Functionalized carbon dots for corrosion protection: Recent advances and future perspectives

1.   Introduction

Corrosion is a growing concern due to several global issues, such as high carbon emissions, resource waste, materials degradation, and environmental pollution. Consequently, corrosion protection technology has received extensive research attention for many industrial applications [1–8]. Corrosion is especially exacerbated in the presence of corrosive ions, such as Cl⁻, and thus, is a huge threat to infrastructure and human lives in such conditions. Therefore, it is necessary to develop effective strategies to eliminate/mitigate metal corrosion. However, since corrosion cannot be completely eliminated, the primary goals of corrosion protection techniques are to reduce the kinetics of corrosion and promote anticorrosion mechanisms [9–15]. Corrosion inhibitors and protective coatings are among the most common and cost-effective methods to protect metallic materials against corrosion degradation [16–21].

Carbon dots (CDs) are a new class of zero-dimensional carbon materials with many desirable properties such as an abundance of raw materials, low toxicity, good water solubility, excellent optical properties, good compatibility, ease of chemical modification, and cost-effectiveness [22–24]. Researchers have deepened their understanding of CDs with recent investigation and exploration on them. Furthermore, they have excellent prospects for applications in diverse fields such as optoelectronic devices, sensors, catalysis, nanocarriers, biological imaging, and tumor treatment. CDs have already been demonstrated to be effective in the field of corrosion protection [25–26]. Therefore, they have received significant interest in the corrosion protection field as potential corrosion inhibitors and fillers in anticorrosion coatings. CDs can form uniformly adsorbed, corrosion-inhibiting films on steel surfaces due to the presence of electronegative atoms and multiple bonds on their surfaces [27–28]. Besides, CDs can undergo facile surface modifications to form carboxyl, amino, carbonyl, and hydroxyl groups that enhance their corrosion inhibition abilities. In addition, the functional groups on the surface of CDs can interact with Fe³⁺ and H⁺ ions, causing a modification to their fluorescence behavior. Therefore, monitoring the generation of these ions during corrosion can help in the early detection of corrosion [29–31]. Thus, CD-based materials have good prospects for the corrosion protection applications as illustrated in Fig. 1 [32–33].

Fig. 1.  Schematic showing the characteristics and applications of functionalized CDs.

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CDs with small particle sizes, large specific surface area, and diverse functional groups can be dispersed easily and uniformly in a polymer matrix. Furthermore, diverse functional groups on the CDs enable strong interactions with the polymer through the covalent bonds formed between CDs and polymer chains, hydrogen bonds formed between functional groups, and van der Waals forces between the individual CDs [34–37]. CD-based coatings can create efficient barriers that slow down the corrosion of metal substrates or suppress crack or local damage propagation due to strong interactions between CDs and polymers, resulting in its self-healing behavior [38]. Furthermore, CDs can be incorporated into coatings to monitor the corrosion underneath metal substrate because of their sensitivity for Fe³⁺ and H⁺. When a coating is damaged, the exposed metal substrate starts to corrode, leading to a change in the local pH. This causes the generation of Fe³⁺ during iron and steel corrosion. The CDs released from the coating matrix then exhibit a change in the fluorescence behavior (obvious fluorescence enhancement/quenching or fluorescence peak shift), indicating the occurrence of corrosion and thus providing an efficient means for nondestructive corrosion detection [39–42].

In this review, we have briefly discussed the preparation methods, physical and chemical properties, and the self-healing/self-reporting applications and mechanisms of CDs in the context of corrosion protection systems. Recent advances in CD-based materials such as corrosion inhibitors, self-reporting fluorescent indicators, and coating fillers are discussed, with a focus on optimization strategies, including surface functionalization and heteroatom doping. The challenges and prospects for the development of CDs are also discussed to promote innovative ideas for further exploration of CD-based anticorrosion systems.

2.   Synthesis and modification of CDs

2.1   Synthesis of CDs

Researchers have developed a series of methods to synthesize CDs that are inexpensive, environmentally friendly, and simple. These methods are broadly classified into two types: top-down and bottom-up [43–48]. The top-down methods employ physical, chemical, and electrochemical pathways to exfoliate or crack large carbon structures into nanosized CDs. The bottom-up methods employs small organic molecules or aromatic compound precursors to prepare CDs via combustion, heat treatment, and microwave treatment [49–51].

2.1.1   Top-down methods

(1) Arc discharge method.

The arc discharge method can increase the speed of the reaction completion by creating a strong current and a high temperature under specific voltage conditions while using gaseous charged particles as conductors. Arc discharge is the earliest known method for producing CDs. In 2004, Xu et al. [52] synthesized single-walled carbon nanotubes using the arc discharge process; then they employed electrophoresis to purify each component in the product and discovered CDs in it. Dey et al. [53] used the arc discharge method to produce highly crystalline B and N co-doped CDs with a narrow size distribution in the range of 4–6 nm. Although the CDs prepared by the arc discharge method typically have small particle size and high oxygen content, the inherent drawbacks such as harsh preparation conditions, complex purification process, and low yield (usually less than 10%) limit the widespread use of this method.

(2) Laser ablation method.

Laser ablation is one of the earliest methods employed to prepare CDs. CDs can be obtained by the interaction of pulsed laser and solid carbon-based materials. Sun et al. [37] took graphite and clay as carbon targets and obtained carbon nanoparticles without fluorescence using laser ablation. After the passivation of products by organic molecules via surface acid treatment and aminated polyethylene glycol, the obtained CDs exhibited bright fluorescence with a quantum efficiency of 4%–10%. Hu et al. [54] produced CDs using a laser with a wavelength of 1.064 mm to irradiate carbon-based materials suspended in organic solutions. First, a small amount of graphite powder was dispersed in polyethylene glycol. The powder was then exposed to a pulsed laser, which after suspending in a suitable medium, was separated and purified to yield CDs. However, the yield and purity of thus prepared CDs are typically low, and the particle size distribution is uneven, making it unsuitable for large-scale and tunable synthesis.

(3) Electrochemical methods.

Electrochemical methods can produce CDs by electrolyzing carbon materials, such as carbon nanotubes, graphite, and others. Zhou et al. [55] created CDs for the first time using multiwall carbon nanotubes as the working electrodes and performed cyclic voltammetry to obtain CDs with an average particle size of ∼2.8 nm. Additionally, Bao et al. [56] proposed a new method for the controlled fabrication of CDs by etching carbon fibers. A bundle of carbon fibers was electrochemically etched at a constant potential for several hours in acetonitrile containing 0.1 M tetrabutylammonium perchlorate (TBAP). The acetonitrile was then evaporated to collect the CDs. Electrochemical methods are relatively inexpensive, simple to operate, and promise mass production.

2.1.2   Bottom-up methods

(1) Hydrothermal synthesis method.

Hydrothermal synthesis uses water or organic solvents as the reaction solvent in an autoclave at a reaction temperature of 100–1000°C and pressure of 1–100 MPa. Peng and Travas-Sejdic [57] first reported the hydrothermal synthesis of fluorescent CDs in 2009. They dehydrated carbohydrates with concentrated sulfuric acid to form carbonaceous species, which were then oxidized using nitric acid. Finally, these carbonaceous species were modified with nitrogen-containing groups to develop blue-emitting CDs. However, CDs produced by this method had a low quantum yield and a non-uniform size distribution. Another report described the use of monk fruit as a carbon source to produce CDs using hydrothermal synthesis [58]. As shown in Fig. 2, monk fruit was heated in a high-pressure reactor for 6 h at 180°C. The color of the resulting solution changed from pale yellow to dark brown when it was cooled to room temperature. CDs with blue fluorescence were finally obtained after dehydration, polymerization, and carbonization. In 2020, Wang et al. [59] synthesized CDs using the hydrothermal method with citric acid and o-phenylenediamine as carbon sources. This was the first successful preparation of CDs with full-spectrum emission to be reported without changing the carbon source and solvent. Using this method, carbon source can be carbonized in a single step to produce CDs with high fluorescence quantum yield and various functional groups. As compared to other preparation methods, hydrothermal method is relatively simple, and the particle sizes of the prepared CDs are fairly uniform.

Fig. 2.  Schematic of the preparation and formation mechanism of N-CDs (where hν is the light energy, and FL is the fluorescence) [58]. Reprinted from Sens. Actuators B, 246, R. Atchudana, T.N.J.I. Edison, D. Chakradhar, et al., Facile green synthesis of nitrogen-doped carbon dots using Chionanthus retusus fruit extract and investigation of their suitability for metal ion sensing and biological applications, 497-509, Copyright 2017, with permission from Elsevier.

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(2) Microwave-assisted method.

Microwave-assisted methods use microwaves to rapidly heat the reagents for polymerizing and carbonizing the reactant monomers into CDs. Tang et al. [60] used a microwave-assisted hydrothermal synthesis with glucose as carbon source to produce CDs with short emission wavelengths (Fig. 3). Wang et al. [61] used carbohydrates (glycerin, ethylene glycol, glucose, sucrose, etc.) and a small quantity of inorganic ions ($\text{PO}_4^{3-}$) as raw materials to create CDs with excellent luminescence properties in few minutes using a microwave-assisted method, thereby eliminating the need for surface passivation. Li et al. [62] prepared CDs in a household microwave oven with a power of 700 W for 10 min and using guanine and ethylenediamine as carbon sources and deionized water as solvent. A dialysis bag was then used to filter out large impurities, resulting in an aqueous solution of CDs with excellent water solubility.

Fig. 3.  Schematic of CD synthesis using microwave method. Reprinted with permission from L.B. Tang, R.B. Ji, X.K. Cao, et al., ACS Nano, 6, 5102–5110 (2012) [60]. Copyright 2012 American Chemical Society.

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(3) Template method.

Template method employs a ductile and inexpensive substance as the reaction template, and the carbon source is attached either to the template surface or inside its cavities using appropriate chemical or physical techniques. Further, the template is removed to obtain the target CDs. Liu et al. [63] used porous silica microspheres as templates and phenolic resins with molecular weights less than 500 as the carbon source to synthesize CDs with a diameter of 1.5–2.5 nm. The silica microspheres were modified with amphoteric polymer F127 using an aqueous route. Subsequently, the silica carrier was treated at a high temperature and centrifuged to produce nanosized CDs; moreover, centrifugation prevented the aggregation of CDs during the pyrolysis process. The CDs thus obtained were etched under alkaline conditions to form micelles through hydrogen bonding, providing an aqueous solution of blue-fluorescing CDs.

2.2   Modification of CDs

Due to the abundance of reactive bonds on the CDs’ surfaces, their structure, morphology, and composition can be easily modified to meet various application requirements. The approaches to the modification of CDs can be divided into three categories: surface functionalization, heteroatom doping, and compositing.

2.2.1   Surface functionalization

Surface functionalization is a powerful method to expand the selectivity and improve the sensitivity of CDs, where various functional groups are attached to CDs through covalent or noncovalent interactions based on the original structure of the CD surface. Shen et al. [64] prepared two types of CDs and discovered that the quantum yield of CDs passivated with polyethylene glycol was twice that of the CDs without passivation. Anilkumar et al. [65] cross-linked the passivator oligomeric poly(ethylene glycol) diamine with pre-prepared CDs to develop giant fluorescence probes containing one or multiple dots with a stable structure and considerably increased fluorescence intensity. Liu et al. [66] used microwave synthesis to create luminescent CDs, where their surfaces are passivated with branched polyethyleneimine (PEI). Being a nitrogen-rich compound, PEI could improve the fluorescence properties of CDs. Liao et al. [67] used a hydrothermal carbonization method to prepare CDs with blue-green fluorescence (λ_em = 510 nm, where λ_em represents fluorescence emission wavelength) and then functionalized them with spiropyran to change their fluorescence emissions to red (λ_em = 650 nm). This phenomenon can be explained by fluorescence resonance energy transfer between spiropyran and CDs under ultra violet (UV) irradiation, causing the emission wavelengths of the functionalized CDs to be red-shifted.

2.2.2   Heteroatom doping

Since heteroatom doping can effectively adjust the surface defects and electronic energy levels of CDs, it is considered to be a new field of carbon-based nanomaterials. Various synthesis strategies have been used to manufacture heteroatom-doped CDs. Fig. 4 shows the timeline of recent developments in heteroatom-doped CDs [68]. Wei et al. [69] prepared a series of nitrogen-doped CDs via Maillard reaction between glucose and amino acids. Surprisingly, the as-prepared CDs exhibited bright photoluminescence, with quantum yields as high as 69.1%. The physical and chemical properties of these CDs, such as size, composition, emission color, surface charge, and lipophilicity, could be easily controlled by varying the side chains of the starting amino acid. Sun et al. [70] used similar methods to prepare nitrogen-doped and nitrogen-sulfur co-doped CDs and found that co-doping with nitrogen and sulfur increased their solubility in water and the size and surface states of co-doped CDs were more uniform and stable.

Fig. 4.  (a) Main approaches for the synthesis of single-atom-doped CDs and hetero-atom-doped CDs. (b) Timeline showing recent progress regarding hetero-atom-doped CDs [68]. Reprinted from Nano Today, 33, S.H. Miao, K. Liang, J.J. Zhu, et al., Hetero-atom-doped carbon dots: Doping strategies, properties and applications, 100879, Copyright 2020, with permission from Elsevier.

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2.2.3   Compositing

In addition to the aforementioned modification methods, CDs can be compounded with noble metals, semiconductor materials, molecular sieves, metal–organic frameworks (MOFs), etc. [71–72]. Liu et al. [73] introduced CDs into molecular sieve matrices in situ under solvothermal conditions using triethylamine as a template, as shown in Fig. 5(a). The molecular CDs@AlPO-5 (aluminophosphate zeolite) composite thus obtained had a quantum yield of up to ~52% and ultra-long lifetimes of up to 350 ms at room temperature and ambient conditions. The fluorescence quantum efficiency and the thermally-induced delayed fluorescence lifetimes of the composites can be regulated by changing the preparation conditions such as the organic templates and solvents. Dong et al. [74] created CDs@Eu-MOF composites by encapsulating pre-prepared CDs into hydrothermally synthesized Eu-MOFs. The composite only exhibited red fluorescence because the CDs were aggregated in pure organic solvent. Alternatively, the CDs were released from CDs@Eu-MOFs in an aqueous solution and emitted intense blue light, while the red fluorescence emitted by Eu-MOFs was quenched due to O–H vibration (Fig. 5(b)). Thus, CDs@Eu-MOFs composites can be used to detect water content in organic solvents due to their unique fluorescence properties.

Fig. 5.  (a) Process involved in the synthesis of photoluminescent CDs@AlPO-5 composite [73]. (b) Synthesis of Eu-MOFs/N, S-CDs and determination of water content in organic solvents. (b) Reprinted with permission from Y.Q. Dong, J.H. Cai, Q.Q. Fang, et al., Anal. Chem., 88, 1748-1752 (2016) [74]. Copyright 2016 American Chemical Society.

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3.   Application of CDs as corrosion inhibitors and fillers in self-healing coatings

3.1   CDs as corrosion inhibitors

Metallic materials are vulnerable to corrosion during their usage and storage. CDs can play a role in their corrosion protection because CDs have electronegative atoms and multiple functional groups on their surfaces, which can improve the uniform adsorption of CDs on the metal surface through ionic bonds. The adsorbed film can minimize the direct contact between the corrosive solution and the metal substrate, resulting in effective corrosion inhibition [75–79].

In general, substances containing nitrogen or sulfur may be used as precursors for the production of CD inhibitors [80–83]. Cui et al. [84] reported the corrosion-inhibiting effect of N-doped CDs (N-CDs) in 2017. The N-CDs were prepared using 4-aminosalicylic acid as the precursor by a solvothermal method, and their corrosion inhibition behavior on Q235 carbon steel was investigated in a 1 M HCl solution. The inhibition efficiency (IE) of 100 mg/L N-CDs was calculated to be 87.2% and 96.0% using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) analyses, respectively. Similarly, in a 0.5 M H₂SO₄ system, similar CDs demonstrated excellent corrosion inhibition on pure copper surfaces. Surface modification and functionalization of CDs without the dopant of heteroatoms (such as nitrogen or sulfur) can also produce CD-based corrosion inhibitors [81,85–87]. For instance, Ye et al. [88] synthesized a green and effective corrosion inhibitor from functionalized CDs (FCDs) through the conjugation of imidazole with citric acid CDs (CA-CDs). The FCDs were employed to protect Q235 carbon steel from a 1 M HCl solution. As shown in Fig. 6, the FCDs-protected steel sample had the lowest corrosion rate as compared to the blank steel, CA-CDs, or imidazole-containing samples, demonstrating excellent corrosion inhibition by FCDs. The primary reason for this good corrosion inhibition was the formation of a protective film via physical and chemical adsorptions.

Fig. 6.  Corrosion rate of Q235 steel in various test solutions: (a) different corrosion inhibitor concentrations after 48 h of immersion and (b) 200 mg/L corrosion inhibitors after different immersion durations [88]. Reprinted from J. Mater. Sci. Technol., 35, Y.W. Ye, D.P. Yang, and H. Chen, A green and effective corrosion inhibitor of functionalized carbon dots, 2243-2253, Copyright 2019, with permission from Elsevier.

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Another method for producing doped CDs is the simultaneous reaction of a carbon source and a heteroatomic source. A series of N-CDs or N, S co-doped CDs (N, S-CDs) were created in this manner, using hydrothermal [89–95]. In Fig. 7, the Tafel curves of carbon steel in 1 M HCl and 3.5wt% NaCl solutions containing varying amounts of N-CDs derived from methacrylic acid and ethyl(methyl)amine are shown. Whether in an acidic or a neutral environment, the corrosion current density (i_corr) gradually decreased with the addition of N-CDs, indicating suppression of corrosion. The IE of steel reached 93.93% (1 M HCl) and 88.96% (3.5wt% NaCl) at 200 mg/L of N-CDs, revealing a high degree of protection for steel in all test environments [91]. Surface analysis demonstrated that N-CDs could form an adsorbed film on the steel surface via physicochemical interaction, reducing its vulnerability to attack by corrosive species.

Fig. 7.  Tafel curves of carbon steel containing various concentrations of N-CDs in (a) HCl and (b) NaCl solutions [91]. Reprinted from J. Mater. Sci. Technol., 43, Y.W. Ye, Z.L. Jiang, Y.J. Zou, et al., Evaluation of the inhibition behavior of carbon dots on carbon steel in HCl and NaCl solutions, 144-153, Copyright 2020, with permission from Elsevier.

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Apart from steel, the corrosion inhibition effect of N or S-doped CDs was also investigated for magnesium and copper alloys in neutral or acidic solutions [24,96–99]. Zhang et al. [24] employed a combination of electrodeposition and immersion coating to prepare a composite coating of N-CDs and polydopamine (PDA) on the surface of a magnesium alloy (AZ91D). The effect of particle size of N-CDs on the corrosion performance and self-healing effect was studied by potentiodynamic polarization and EIS analyses. The study revealed that the corrosion resistance of N-CD/PDA composite coating was enhanced with an increase in the N-CD particle size. In addition, the N-CD/PDA composite coating also exhibited self-healing behavior. Overall, these findings suggested that N-CDs/PDA composite coatings could significantly improve the corrosion resistance of magnesium alloys. This opens up a new direction for the design of protective coatings for metals.

The protection of copper alloys from corrosion using CDs has also been reported. Recently, Zeng et al. [12] synthesized a new corrosion inhibitor with Ce and N co-doped CDs (Ce@N-CDs), which is green and efficient, through a hydrothermal method. Their results showed that Ce-10% (200 mg/L) had the best inhibition effect with a high IE of 98% for copper in 1 M HCl solution. After corrosion, Ce@N-CDs formed a dense adsorption/passivation film on the copper surface. The adsorption film was mainly formed due to the pairing of unsaturated bonds between N-CDs and the copper surface, while the passivation film was mainly composed of oxide formed from the reaction between Ce and the surrounding environment. Their dense structure can effectively inhibit erosion by corrosive media.

Zhang et al. [98] synthesized novel N, S-CDs using o-phenylenediamine (o-PD) and thiourea as precursors and evaluated their corrosion inhibition performance for copper in a 0.5 M H₂SO₄ solution at various temperatures. Notably, the as-prepared N, S-CDs were first used as copper corrosion inhibitors with excellent corrosion resistance in H₂SO₄ solution. As shown in Fig. 8, the low-frequency impedance moduli of the sample at 0.01 Hz (|Z|_0.01Hz) with 50 mg/L of N, S-CDs at 298, 308, and 318 K were higher than those of the blank solution by more than two orders of magnitude. Although the IE of N, S-CDs decreased at higher temperatures, it remained as high as 89.4% at 318 K, indicating its excellent inhibition performance. Considering the experimental data and theoretical understanding, it can be concluded that the interaction between the protective layer of N, S-CD and copper was mainly chemisorption.

Fig. 8.  (a) Bode diagram of copper immersed in 0.5 M H₂SO₄ solution at different temperatures; (b) bode diagram of copper immersed in 0.5 M H₂SO₄ solution containing 50 mg/L N, S-CDs at different temperatures; (c) adsorption and anticorrosion mechanism of N, S-CDs on copper matrix in acid medium [98]. Reprinted from J. Mol. Liq., 338, Y. Zhang, B.C. Tan, X. Zhang, et al., Synthesized carbon dots with high N and S content as excellent corrosion inhibitors for copper in sulfuric acid solution, 116702, Copyright 2021, with permission from Elsevier.

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Because of the excellent corrosion inhibition and eco-friendliness of rare-earth elements, novel Ce@N-CDs were synthesized to protect Q235 carbon steel in 1 M HCl solution [100]. Q235 steel immersed in Ce@N-CDs solution exhibited lower corrosion current density as compared to the steel sample immersed in the blank solution [101–102]. When the concentration of Ce@N-CDs was 200 mg/L, the IE increased to 96.4%. In this work, the homogenously distributed Ce@N-CDs corrosion inhibitors significantly improved the electrochemical resistance of Q235 carbon steel, indicating the good protective effect on metal samples in a corrosive medium. The protective effect was mainly attributed to the triple action of electrostatic adsorption, chemical adsorption, and the presence of Ce-containing ligand complexes on the steel surface (Fig. 9).

Fig. 9.  Schematic of the corrosion protective mechanism of Ce@N-CDs inhibitor [100]. Reprinted from J. Mol. Liq., 349, Z.X. Liu, X.Y. Hao, Y. Li, et al., Novel Ce@N-CDs as green corrosion inhibitor for metal in acidic environment, 118155, Copyright 2021, with permission from Elsevier.

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In comparison to the preparation of CDs using citric acid, thiourea, imidazole, and other chemical products as precursors, the use of natural plants as CD precursors has the advantage of being cheap, eco-friendly, and easily available [103–105]. Anindita et al. [106] used a simple heating approach to create CDs derived from durian juice. Because of the presence of sulfur-based compounds in durian, there were abundant sulfur-based groups on the CDs, which helped achieve strong adsorption on the copper substrate. However, the corrosion inhibition efficiencies of these CDs, as determined by weight loss and potentiodynamic polarization, were only 73.5% and 85.8%, respectively, much lower than that of CDs prepared using standard chemicals.

Recently, research has shown that the functionalized CDs have broad prospects for application as metal corrosion inhibitors. Many studies have reported the application of functionalized CDs for corrosion protection in materials such as steel, copper, and magnesium alloys. Moreover, CDs have also been utilized in the development of smart protective coatings with self-healing and/or self-reporting properties, which can effectively detect and repair damaged coatings in real time. Therefore, the design of green, inexpensive CD corrosion inhibitors with high IE and low risk of environmental pollution is of great significance for the future [87,89,107]. Table 1 summarizes the preparation methods, dopant(s), and the corrosion IE of CDs in different media, which are reported in recent years [12,81–84,88,90–94,96–100,106,108–109].

Table  1.  Preparation of CDs and their corrosion IE in different media

Materials	Dopant(s)	Concentration / (mg∙L−1)	Medium	Metal substrate	ηWL / %	ηPP / %	ηimp / %	Ref.
Cerium nitrate & ammonium citrate-based CDs	N, Ce	200	1.0 M HCl	Copper		98		[12]
Ammonium citrate-based CDs	N	200	1.0 M HCl	Q235 carbon steel		97.4	96.9	[82–83]
4-Aminosalicylic acid (ASA) based CDs	N	100	1.0 M HCl	Q235 carbon steel		87.2	96.0	[84]
	N	50	0.5 M H2SO4	Copper		84.7	91.1	[81]
Citric acid-based CDs functionalized with imidazole	N	200	1.0 M HCl	Q235 carbon steel		94.0	93.4	[88]
Methacrylic acid & n-butylamine-based CDs	N	200	1.0 M HCl	Q235 carbon steel		95.0		[90]
Methacrylic acid & ethyl(methyl)amine-based CDs	N	200	1.0 M HCl	Q235 carbon steel		93.9		[91]
	N		3.5wt% NaCl			89.0
Citric acid & L-histidine-based CDs	N	200	0.1 M HCl	Q235 carbon steel		96.1		[92]
Citric acid & ethylenediamine-based CDs	N	100	1.0 M HCl	Q235 carbon steel		92.1	93.7	[93]
O-phenylenediamine (o-PD) & folic acid-based CDs	N	150	1.0 M HCl	Q235 carbon steel		91.3	95.4	[94]
Glucose & 4-amino-3-hydrazine-5-mercapto-1,2,4-triazole-based CDs	N, S	70	3.5wt% NaCl	Copper		83.1	83.4	[96]
Citric acid & L-serine-based CDs	N	200	0.5 M H2SO4	Copper		77.3	93.6	[97]
o-PD & thiourea-based CDs	N, S	50	0.5 M H2SO4	Copper		98.5	99.8	[98]
Citric acid & polyethyleneimine branched (PEI)-based CDs	N	100	0.5 M H2SO4	Copper		87.6	96.1	[99]
Citric acid & Ce(NO3)3-based CDs	Ce, N	200	1.0 M HCl	Q235 carbon steel		96.4		[100]
Durian juice-based CDs	S	800	1wt% NaCl	Copper	73.5	85.8		[106]
Citric acid & urea-based CDs	N	30	0.5 M H2SO4	Q235 carbon steel		97.8		[108]
Citric acid & thiourea-based CDs	N, S	400	1.0 M HCl	Q235 carbon steel		96.6	94.6	[109]
Note: ηWL, ηPP, and ηimp represent the corrosion IE calculated using three test methods: weightlessness test, potentiodynamic polarization test, and impedance modulus test, respectively.

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3.2   CDs as fillers in self-healing coatings

During these decades, the development of self-healing coatings has received considerable interest for intelligent corrosion control. In addition to being corrosion inhibitors, CDs can be incorporated into anticorrosion coatings to achieve self-healing behavior [93,110]. Self-healing coatings can be classified into intrinsic and extrinsic ones. In intrinsic self-healing, coatings are repaired by restoring the intrinsic chemical bonds and/or physical conformation in the polymer matrix [111–112]. One of the most direct methods of extrinsic self-healing is to coat the existing coatings with a polymerizable repair agent to form a protective film and restore the barrier function and integrity. Furthermore, corrosion inhibitors can be added into extrinsic self-healing coatings, which form a complex with metal ions through adsorption or chemical reaction to inhibit the corrosion of metal substrate [113–114].

In a recently reported research, functionalized CDs have been added into coatings and are combined with extrinsic and intrinsic healing mechanisms. On the one hand, when CDs are added to the coatings as fillers, their surface functional groups (hydroxyl, carboxyl, amino, etc.) can strongly interact with the polymer coating to achieve self-healing of local pores/cracks [115–116]. On the other hand, CDs can be easily adsorbed on the metal surface to form a protective film by physical and chemical adsorptions, thereby preventing the corrosive medium from coming into contact with the metal to inhibit metal corrosion. The self-healing properties of functionalized CDs have been widely applied in intelligent anticorrosion coatings, with broad application prospects; here, along with the effective prevention of metal corrosion, damages are repaired themselves, thus extending the service life of metals [117–119].

Studies confirmed that CDs with dimensions of 5 nm (CDs-5) can be blended into polyurethane (PU) to achieve self-healing behavior [28]. CDs-5 were generated using a one-pot solvothermal synthesis with –OH, –COOH, –NH₂, and –C=O–NH₂ groups. CD/PU composites with a CDs-5 concentration of 2.61 mg/mL are referred to as CDs-5/PU-3. The scratch profiles (depth and width) of blade-scratched CDs-5/PU-3 ribbon were characterized while stored at room temperature (Fig. 10(a)). As shown in Fig. 10(b)–(c), within 48 h of storage, the scratch on CDs-5/PU-3 gradually shrank and eventually disappeared. The self-healing process was driven by the interfacial bonding (covalent bonding, hydrogen bonding, and van der Waals force) between CDs and the polymer matrix, and the process can be optimized by modifying the functionality of the CDs. Additionally, by submerging in an aqueous CDs-5 solution for a few seconds, the tensile strength of the healed CDs/PU composite ribbons could also be increased back to 70% of that of the pristine ribbons. This could be attributed to the bonding abilities of CDs (Fig. 10(d)). As shown in Fig. 10(e), the corrosion potential (E_corr) of pure PU coating was approximately −0.56 V, and the corrosion current (I_corr) was ∼3.07 × 10⁻⁶ A. In comparison, the E_corr of CDs-5/PU-1 increased positively to −0.139 V, and I_corr decreased to 1.45 × 10⁻⁸ A (CDs-5/PU-1 refers to CD composites and coatings with a CDs-5 concentration of 0.95 mg⋅mL⁻¹), suggesting a decrease in the probability and rate of corrosion. The electron transfer number (n) of CDs-5 was determined from the slope of Koutecky–Levich plots and is found to be 3.8. This shows that the more effective, four-electron pathway, which reduces oxygen to H₂O in the oxygen reduction reaction (ORR), is attributed to the small n (Fig. 10(f)). CDs-5 have a high specific surface area due to their nanoscale dimensions and terminal surface functional groups (–NH₂, –COOH, –OH, and –CONH₂). The connection and interaction between adjacent CDs-5 lead to the self-healing of local pores/cracks, thereby reducing the formation, expansion, and propagation of pores and capillaries. Therefore, CDs-5 effectively block the crossing over of oxygen and water through the defect paths. In addition, the hydrophilic functional group on the surface of CDs-5 captures oxygen while adsorbing H₂O. The O₂ molecule adsorbed on the surface of CDs-5 is directly reduced to H₂O through the four-electron pathway (Fig. 10h), thus inhibiting the corrosion process.

Fig. 10.  Self-healing and corrosion resistance of CDs-5/PU: (a) profiles with deep cracks introduced into CDs-5/PU-3 after being scratched and after self-healing at room temperature for 48 h; (b, c) width and depth time evolution of the fracture detected by an Alpha step profilometer of CDs-5/PU-3 and PU self-healed at room temperature; (d) tensile strength of original CDs-5/PU-3 ribbons and healed CDs-5/PU-3 cut ribbons with different thicknesses; (e) Tafel curves of CDs-5/PU-1 composite and PU without CDs (where I represents the current); (f) LSV curves of CDs-5 at different rotation speeds (800–2000 r/min) and j⁻¹ versus ω^−1/2 graphs showing the relationship between current density and the rotation of the disk under different electrode potentials (where j⁻¹ represents the inverse current density, and ω^−1/2 represents the inversed square root of angular velocity); (g–h) schematic of the transmission path of O₂ and H₂O on the surface of a CD-5 through the CDs-5/PU and ORR process. C. Zhu, Y.J. Fu, C.G. Liu, et al., Adv. Mater., 29, 1701399 (2017) [28]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.

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Another self-healing strategy involves the use of external fillers containing healing reagents or corrosion inhibitors. Doped CDs have been added directly into coatings [120] or combined with other fillers, such as polycarbazole [121], ZnO [122], and graphene [110], to improve their self-healing ability. Ding et al. [123] investigated the ability of CDs to promote the self-healing of coatings via the corrosion inhibition effect. CD-functionalized MXene (CM) nanosheet was prepared and added to an epoxy matrix via a bioinspired, self-aligned process and random dispersion. As shown in Fig. 11, the randomly dispersed CD-Ti₃C₂T_X-EP (R-CM-EP) had lower corrosion resistance than the blank, scratched epoxy due to direct contact between MXene and the metal substrate, which accelerated the electrochemical reactions. EIS results showed that the prepared CD-Ti₃C₂T_X-EP (B-CM-EP) had a higher corrosion protection effect. This indicates that good dispersion of the CM nanosheets could improve the barrier property of the coating resin and suppress electrochemical reactions. Furthermore, the CDs with heteroatoms could be adsorbed onto metal substrate to prevent corrosive ions from reaching the metal surface. The corrosion morphologies without visible corrosion products (Fig. 11(i)) demonstrated that CDs inhibited the corrosion process.

Fig. 11.  Tafel plots of (a) scratched EP, (b) R-CM-EP, and (c) B-CM-EP coatings in 3.5wt% NaCl solution; (d) their corrosion potential and Tafel parameter plots and (e) corrosion current of scratched coatings plotted at different times; (f) impedance Nyquist plots of scratched EP, R-CM-EP, and B-CM-EP coatings after 48 h of immersion, where lines are the fitting curves; SEM images of scratched coating protected steels: (g) EP, (h) R-CM-EP, and (i) B-CM-EP; corrosion mechanisms of (j) EP, (k) R-CM-EP, and (l) B-CM-EP coatings [123]. Reprinted from Chem. Eng. J., 430, J.H. Ding, H.R. Zhao, and H.B. Yu, Structure and performance insights in carbon dots-functionalized MXene-epoxy ultrathin anticorrosion coatings, 132838, Copyright 2021, with permission from Elsevier.

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Ye et al. [110] used functionalized CDs derived from citric acid derivatives as intercalating agents to modify graphene before dispersing them in epoxy resin to develop a CD-modified graphene/epoxy (CDs-G/EP) coating. The structure, self-healing property, and corrosion resistance of the coating were then investigated. The functionalized CDs and graphene formed a “π–π interaction,” and CDs could significantly improve the dispersibility and interfacial compatibility of graphene in the coating matrix. Because of the high physical barrier of dispersed graphene and the corrosion inhibition ability of CDs, the water absorption by CDs-G_0.5%/EP coating (the composite coating with 0.5wt% CDs-G addition) after 50 d of immersion was only 4.4%. This implies that, compared with pure epoxy coatings, the oxygen permeability and water absorption of the coating were significantly reduced.

Wan et al. [35] utilized amino-functionalized carbon quantum dots (FCDs) as intercalation agents to extract and modify boron nitride, leading to the formation of FCD nanocomposites and boron nitride nanosheets (BNNS). The FCDs/BNNS were then dispersed in a waterborne epoxy (WEP) matrix to create composite coating using the spray method. This WEP FCDs/BNNS composite coating had exceptional corrosion resistance due to the synergistic effect of BNNS as a physical barrier and the corrosion inhibition effect of FCD. This study highlighted the potential of FCDs as an effective intercalation agent for the modification of BNNS and the development of advanced composite coatings with improved corrosion resistance, which might have significant implications for the development of protective coatings for various industrial applications.

4.   Application of CDs as corrosion indicator in self-reporting coatings

Self-reporting coatings can detect coating damages and metal corrosion in their early stages. Additionally, they have the potential to significantly improve the safety and service lives of materials used in a wide range of industrial applications. When the coating is damaged and corrosion commences on the underlying metal substrate, the indicator is released and activated by triggers such as pH changes or the appearance of corrosion-generated metal cations due to corrosion and other physicochemical reactions within localized defects. Moreover, the fluorescence intensity of CDs can change with the degree of damage to the coating, making them a promising tool for the intelligent monitoring of coating damage. Additionally, CDs can be incorporated into coatings to enhance their fluorescence properties and improve the accuracy of damage detection. In this way, self-reporting coatings can express strong fluorescent signals or color changes to highlight coating failures. Due to their excellent photoluminescence properties and diverse surface functional groups, CDs can interact with some target ions (Fe³⁺ and H⁺) to manifest changes in their fluorescence and provide timely notification of any processes of interest, like corrosion, that generate these species. This feature would be very beneficial for self-reporting coatings [124–125]. Similarly, CDs can react with various metal ions (copper, zinc, aluminum, magnesium, and manganese ions) to produce fluorescent effects, thus providing corrosion warning for those alloys too. Therefore, CDs also have potential applications in the field of corrosion self-reporting for copper, magnesium, aluminum, and zinc alloys [126–127].

4.1   Fe³⁺ sensing

Because of the limited amount of metal cations and pH changes at the initial stage of the corrosion process, early and efficient detection of iron/steel corrosion remains challenging. Furthermore, most existing methods (eddy current method, acoustic emission technology, and radiographic technology) require either complicated sample pretreatment or expensive testing equipment [128–129]. Therefore, exploring an efficient, green, and nondestructive monitoring technology for early corrosion detection is urgently required. Recently, CDs have revealed a high potential for detecting Fe³⁺ ions, which can be used in coatings to warn of coating damage and corrosion of the underlying metal substrate [130–134].

Liu et al. [135] used a simple solvothermal method to create bright fluorescent CDs and detected electrochemical corrosion using N-doped CDs with superior Fe³⁺ responsiveness. These blue-emitting fluorescent CDs exhibited characteristic emission bands at 450 nm, as shown in Fig. 12(a)–(b), which were quenched with an increase in Fe³⁺ concentration. The fluorescence intensity of the CDs varied linearly with Fe³⁺ concentration in the range of 10–300 μM (Fig. 12(c)). Quantitative analysis revealed that the CDs’ limit of detection (LOD) for Fe³⁺ was 0.9 μM. Furthermore, metal ions other than Fe³⁺ had very little effect on the intensity of CD emission (Fig. 12(d)), indicating a very selective response to Fe³⁺. The fluorescence quenching of CDs can be attributed to electron transfer caused by the interaction of active organic groups, such as –OH, –COOH, –NH₂, and C=O, on the surface of CDs with Fe³⁺ to form Fe³⁺–CD complexes. The fluorescence quenching of Fe³⁺ by CDs was very obvious and detectable by unaided human eyes. This promising responsiveness of CDs provides a good opportunity for real-time and nondestructive visual detection of Fe³⁺ during electrochemical corrosion.

Fig. 12.  (a) Fluorescence spectra of CDs at different concentrations of Fe³⁺ ($\boldsymbol{C}_{{\bf{Fe}^{\bf3+}}}$，0–1000 μM). (b) Relationship between the ratio of fluorescence intensity change ((I₀ − I)/I₀) and Fe³⁺ concentration at 450 nm, where I₀ and I represent the fluorescence intensity at 450 nm before and after the addition of Fe³⁺. Illustration: images of CD aqueous solution processed with a series of concentrations of Fe³⁺ under visible and UV light. (c) Linear fitting of the ratio of fluorescence intensity change at 450 nm ((I₀ − I)/I₀) versus the concentration of Fe³⁺ (10–300 μM). (d) Fluorescence intensity ratio of CDs at 450 nm (I/I₀) before and after adding Fe³⁺, Fe²⁺ and other metal ions [135]. Reprinted from J. Colloid Interf. Sci., 606, Z. Liu, R.N. Jia, F. Chen, G.L. Yan, W.G. Tian, J.M. Zhang, and J. Zhang, Electrochemical process of early-stage corrosion detection based on N-doped carbon dots with superior Fe³⁺ responsiveness, 567-576, Copyright 2021, with permission from Elsevier.

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Liu et al. [133] prepared CDs with bright fluorescence using citric acid and ammonia as the raw materials by a simple solvothermal method and proposed using these CDs to synergistically inhibit and monitor corrosion. It was found that the active groups on the surface of CDs promoted the formation of an adsorbed film of fluorescent CDs on the iron plate (Fig. 13(a)), which could protect the iron plate from corrosion by acidic media. On the other hand, due to the complexation between CDs and Fe³⁺, the CDs adsorption film can respond effectively to the Fe³⁺ ions released due to corrosion and monitor the degree of corrosion. Fig. 13(b) shows that the iron plate shows strong CD fluorescence before corrosion. However, with the prolonged immersion in 0.1 M HCl, the CD fluorescence on the iron plate surface begins to weaken until it is quenched. These changes in fluorescence intensity can be detected by the unaided eye and can be used as a signal to track the spread of corrosion.

Fig. 13.  (a) Synergistic corrosion inhibition and monitoring mechanism of as-prepared CDs on iron; (b) macroscopic and microscopic changes in fluorescence of iron plate coated with CDs after soaking in 0.1 M HCl [133]. Reprinted from Colloid Surf. A, 650, Z. Liu, R. Jia, Y. Jian, et al., N-doped carbon dots as a multifunctional platform for real-time corrosion monitoring and inhibition, 129499c, Copyright 2022, with permission from Elsevier.

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However, there are not many applications of CDs to provide early warnings of corrosion at present. Yu et al. [38] reported the synthesis of CDs with high detection sensitivity to Fe³⁺; this could be used in self-reporting coatings. Yu et al. [38] used Jinhua bergamot as the carbon source to produce CDs. The prepared CDs had good stability and bright-blue fluorescence at 440 nm with a high quantum yield of 50.78%. The photoluminescence of CDs can be significantly reduced by Tris-HCl buffer solution. The linear range of Fe³⁺ detection achieved using fluorescence emission spectra was 0.025–100 μM. The fluorescence quenching is due to the strong binding affinity and fast chelating kinetics between Fe³⁺ and –NH₂ and –COOH on the CD surfaces, thereby forming Fe³⁺–CD complexes. This method offers a new idea for the development of CD-based Fe³⁺ detection sensors, with the advantages of good selectivity, fast response, low cost, and wide linear range. Zulfajri et al. [26] adopted cranberry beans as raw materials to produce a novel CD-based fluorescent sensor (CB-CDs) using a hydrothermal method without any functionalization or modifications. The synthesis method was nontoxic, chemical-free, simple, and environmentally friendly. CB-CDs had a specific binding force, high selectivity, and good sensitivity to Fe³⁺, which caused aggregation and reduced fluorescence intensity. By changing the excitation wavelength, the CDs emitted a wide fluorescence emission range from 410 to 540 nm and could be used to detect Fe³⁺ ions. It was found that Fe³⁺ had a much stronger quenching effect on the fluorescence of CDs than other metal ions. The detection of Fe³⁺ could be realized within 3 min (Fig. 14(a)–(b)). The spectroscopic data revealed that Fe³⁺ ions could be detected in a concentration range of 30–600 μM, with a very low detection limit of 9.55 μM. As a result, CB-CDs can be used as an environmentally friendly nanomaterial for corrosion monitoring.

Fig. 14.  (a) FL emission spectra of CB-CD solutions doped with various metal ions; (b) CB-CD solutions with different metal ion concentrations of 500 μM under UV irradiation [26]. (c) Graphical illustration of the fabrication and application of BNSCDs [77]. (c) Reprinted from Mater. Sci. Eng. C, 118, S.M. Song, J.H. Hu, M.L. Li, et al., Fe³⁺ and intracellular pH determination based on orange fluorescence carbon dots co-doped with boron, nitrogen and sulfur, 111478, Copyright 2020, with permission from Elsevier.

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Song et al. [77] used a one-step hydrothermal treatment of 4-carboxyphenylboronic acid and 2,5-diaminobenzenesulfonic acid at 200°C for 8 h to create fluorescent boron, nitrogen, and sulfur co-doped CDs (BNSCDs) that respond to multiple target ions/species. Owing to the effective quenching of BNSCDs fluorescence by Fe³⁺ via electron transfer, a selective and sensitive method for Fe³⁺ determination was developed, with a linear range of 1.5–692 μmol/L and a detection limit of 87 nmol/L. Furthermore, the proton transfer-based fluorescent BNSCD had an excellent linear relationship with pH in the range of 1.60–7.00, enabling accurate pH measurements (Fig. 14(c)). As a result, the fluorescence response of CDs to Fe³⁺ and their pH change can be used to detect early metal corrosion [136–138].

Hence, CDs offer a promising opportunity for quick, precise, and visual detection of Fe³⁺ during electrochemical corrosion. The recently investigated Fe³⁺-sensitive performance for various CDs is summarized in Table 2 [77,135,139–152]. However, there is a lack of research on the use of CDs to detect corrosion of metal substrates. Hence, the scientific issue of CDs as corrosion indicators merits special attention.

Table  2.  Comparison of CDs with different methods and materials for Fe³⁺ detection

Methods and materials	Detection range / μM	LOD / μM	Ref.
Nitrogen and sulfur co-doped CDs	1.5–692	0.087	[77]
Bright-blue emitting CDs	10–300	0.9	[135]
Single precursor N-doped CDs	0–1.3	0.066	[139]
Phosphazene-based multicentered sensor	4–30	4.8	[140]
Rhodamine B fluorescent chemosensor	10–100	0.76	[141]
Terthiophene-phenylamine-derived sensor	0–10	0.1	[142]
Supramolecular host-guest system	40–300	0.21	[143]
Agarose/lanthanide coordination hybrid membrane	0.1–2.1	0.1	[144]
ZnO/Cd-based fluorescent sensor	1–20	0.17	[145]
L-glutamic carbon quantum dots	0–50	4.67	[146]
Electrochemical oxidation of graphite electrode	10–200	1.8	[147]
CQDs	1–250	0.52	[148]
Color-tunable N-doped CDs	0–500	0.2	[149]
Highly photoluminescent nitrogen-rich CDs	1.0–10	0.58	[150]
Anthracene-bearing bisdiene macrocycle	50–200	38	[151]
Fluorescein-based porous aromatic framework	5–23	2.2	[152]

 | Show Table

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4.2   pH sensing

During the corrosion, the cathode pH rises while the anode pH falls. Thus, monitoring the local pH changes can also give us information about the progress of corrosion. When a coating is damaged, a galvanic reaction (anodic or cathodic reaction) occurs on the metal surface, which changes the pH in the vicinity of the damaged region. Hence, pH indicators can be used as coating fillers to achieve smart coatings that can self-report in case of damage [153–155]. When the concentration of H⁺ changes, the multiple reactive oxygenated groups on the surface of CDs would produce a strong fluorescent signal, providing a visual signal for coating damage or early corrosion detection in a nondestructive manner [156–157]. As a result, the application of CDs as a pH indicator to corrosion monitoring has a wide range of potential applications.

Liu et al. [158] demonstrated a quantitative relationship between pH-sensitive photoluminescence (PL) spectra and CD surface structure for the first time. This work facilitated the rational design of CD-based pH sensors by quantitatively modulating the carboxyl and β-dicarbonyl groups on the CD surfaces. The combination of carboxylate and H⁺ was considered to affect only the excited state proton transfer process, and the balance of carboxylic acid ↔ carboxylates at different pH values would increase the ratio of nonradiative to radiative attenuation in CDs, resulting in a decrease in the PL intensity (Fig. 15(a)–(b)). Fig. 15(c) shows that PL attenuation of C-dots-h at higher pH is much slower than that at lower pH. The degree of delocalization of the CD-conjugated systems increased with the dissociation of α-H to form enols, leading to a red shift in their emission spectrum. In addition, the curve of PL intensity and pH value of C-dots-h can also be fitted well using the above S-shaped logic function (Fig. 15(d)). When pH is decreased from 7.0 to 1.8, the PL intensity of the C-dots-h decreased with a linear range of 2.0−7.0, indicating that hydrogen bonds could inhibit the dissociation of α-H in β-carboxyl groups, thus reducing the rate of change of PL.

Fig. 15.  (a) Representation of the transformation of C-dots-h structure under different pH conditions; (b) PL spectra of the C-dots-h at different pH values ranging from 1.8 to 12.0; (c) time-resolved PL curves of C-dots-h at different pH values; (d) fitting relationship between C-dots-h intensity and pH value. Reprinted with permission from C. Liu, M.L. Yang, J. Hu, et al., J. Phys. Chem. Lett., 12, 2727-2735 (2021) [158]. Copyright 2021 American Chemical Society.

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Li et al. [159] developed novel red fluorescent CDs (R-CDs) for pH determination (Fig. 16(a)). The obtained R-CDs demonstrated excellent photostability and biocompatibility, as well as good pH sensing ability. At pH values of 5.0 and 9.0, the R-CDs solution exhibited a color change from pink to yellow, as shown in Fig. 16(b)–(c). Fig. 16(d)–(e) depicts the absorption spectra of R-CDs at pH ranging from 3.0 to 12.0. The intensity ratio of the two absorption peaks (A_{520 nm}/A_{436 nm}) gradually decreased with an increase in pH, demonstrating an excellent linear relationship in the pH range of 5.0–7.4. Furthermore, the pH sensitivity of R-CDs was reversible (Fig. 16(f)). As shown in Fig. 16(g), the PL intensity of R-CDs did not change significantly with the addition of factors commonly known to cause interference, such as metal cations, anions, and amino acids, indicating that they have excellent chemical stability and hydrogen ion concentration specificity.

Fig. 16.  (a) Schematic of a multifunctional sensor based on red fluorescent R-CDs for opening detection of intracellular TCs level and pH; (b) fluorescence spectrum change of R-CDs when pH value decreased from 12.0 to 3.0 (λ_ex = 538 nm, λ_ex represents fluorescence excitation wavelength); (c) sigmoidal fitting of the fluorescence intensity at 538 nm (the pKa value defines the pH at which certain functional groups are 50% in ionized and 50% in a nonionized form); (d) UV–Vis spectral changes of R-CDs when pH value decreased from 12.0 to 3.0; (e) relationship between the absorption intensity of R-CDs and pH value, Illustration: response linearity; (f) PL repeated cycles from pH 5.0 to 9.0; (g) PL intensity of R-CDs at pH 7.0 in the presence of metal ions, anions, and amino acids [159]. Reprinted from Sens. Actuators B, 332, L. Li, L.H. Shi, J. Jia, et al., Red fluorescent carbon dots for tetracycline antibiotics and pH discrimination from aggregation-induced emission mechanism, 129513, Copyright 2021, with permission from Elsevier.

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Zhang et al. [160] synthesized pH-responsive CDs with red emission (R-CDs) using citric acid and urea as precursors. The fluorescence intensity of R-CDs gradually increased as the pH value of the solution increased from 1 to 12; moreover, the emission peaks of R-CDs in different pH solutions were slightly different. The study summarized the normalized PL intensities at different pH conditions. The PL intensities presented linear changes in two similar exponential growths in acidic and alkaline environments, indicating that R-CDs could be expected to detect pH values in the range of 1–12. The pH-dependent changes in fluorescence intensity enable continuous pH monitoring in practical applications for real-time and visual corrosion inspection.

5.   Summary and outlook

In summary, CDs have a wide range of applications as corrosion inhibitors and coating fillers in self-healing and self-reporting coatings due to their corrosion-inhibiting properties, fluorescence, low toxicity, ease of chemical modification, and cost-effectiveness. Herein, the synthesis methodology, physical and chemical properties, and anticorrosion mechanisms of CDs are discussed. However, further work is needed to optimize the properties of functionalized CDs. Moreover, efforts to develop CDs for corrosion protection may place a considerable emphasis on the following aspects.

(1) Although doped CDs have shown great promise as green corrosion inhibitors, nitrogen (o-PD, ethylenediamine) or sulfur sources (thiourea) used in many studies are not environmentally friendly. Hence, more ecofriendly alternatives need to be explored. In addition, there is a need to develop CD-based materials with lower costs and higher yields to increase competitiveness in the field of anticorrosion coatings.

(2) CD coatings can have intrinsic or extrinsic self-healing behavior due to their ease of chemical modification and excellent corrosion inhibition. These two aspects could be combined to further improve the self-healing performance of the coating system. Furthermore, it is necessary to explore more solvents as CD dispersants to further improve their dispersibility in anticorrosion coatings and interfacial compatibility between fillers and resin matrices.

(3) The detection of Fe³⁺ and pH by CDs presented in the current study was primarily geared to applications in biological and sensing fields. However, a large amount of work still needs to be done on CDs for early corrosion warning systems. Hence, the application of CDs as Fe³⁺ and pH indicators should be further developed to achieve nondestructive testing and highly sensitive detection of coating failures and initiation of corrosion.

(4) The work of combining corrosion inhibition and fluorescence characteristics of CDs needs to be explored urgently. Novel CD-based coatings with self-reporting and self-healing properties should be developed.

(5) Future work should systematically explore the optimal amount of functionalized CDs added in different coatings and provide sufficient coating performance data to support the research development and application of CDs in anticorrosion coatings.