Sensitive Fluorescent Sensor Research Articles

Rapid environmentally benign label free detection of heparin using highly fluorescent N,S-CDs sensing probe through a turn-on mechanism

Heparin (HEP) is one of the oldest anticoagulant drugs that currently still in widespread clinical use. It lacks chromophore and not easily derivatized due to its hydrophilic nature. In this work we developed a green, selective, and sensitive fluorescence sensor for detection of HEP in its injection dosage forms. The sensor is composed of nitrogen and sulfur co-doped carbon quantum dots (N,S-CDs) semi quenched by Fe3+. The N,S-CDs were prepared using microwave assisted pyrolysis in 3.5 min and exhibited high emission at 425 nm after excitation at 350 nm with high quantum yield of 96%. Owing to the anionic nature of HEP, it could compete with N,S-CDs for Fe3+ complexation resulting in turning-on the quenched fluorescence. This fluorescence enhancement was linear over a concentration range between 6 and 20 μg/mL (R2 = 0.99) with a limit of detection of 1.41 µg/ml. The accuracy and precision of the proposed sensor were indicated by percentage recovery values between 98% −102% and %RSD less than 2, respectively. Furthermore, the proposed sensor was successfully applied for determination of HEP in injection dosage form. The developed sensor showed excellent greenness on analytical eco-scale (score 93%) and GAPI scale.

A facile and sensitive hexahomotrioxacalix[3]arene-based fluorescent sensor for the detection of trace amounts of 2,4,6-trinitrophenol

Nitroaromatic compounds are common explosives and toxic pollutants, the selective and sensitive detection of which is of great importance. Herein, a facile and sensitive fluorescent sensor L was constructed for the sensing of TNP based on the hexahomotrioxacalix[3]arene skeleton. The fluorescence emission of L was drastically quenched in the presence of 2,4,6-trinitrophenol (TNP), while other tested NACs, metal ions, and anions induced negligible changes. Under the optimized conditions, the spectroscopic studies revealed that L exhibited extremely sensitive and selective TNP recognition, with a detection limit of 9.17 × 10−7 M and a quenching constant of 2.44 × 104 M−1. The sensitivity of sensor L for TNP was attributed to the formation of a ground-state charge-transfer complex and an inner filter effect, which also contributed to the special selectivity of the sensor among the various nitroaromatic analogues. Compared with previous reports, L can serve as a highly efficient sensor for the sensing of TNP and can be employed over a wide pH range of 2 to 12. Sensor L was effectively used to quantify TNP in real water and soil samples. Additionally, fluorescent test strips were also developed for visual and rapid detection of TNP in both the solution and vapour phases.

Carbon Dot‐Based Fluorescence Resonance Energy Transfer (FRET) Systems for Biomedical, Sensing, and Imaging Applications

AbstractCarbon dots (CDs) emerge as a potential group of photo‐luminescent nano‐materials due to their excellent optical, electrical, and chemical properties, as well as their competence in a wide range of environmental applications. CDs have unique and appealing properties such as excellent stability, low toxicity, water solubility, and derivability. When coupled with CDs, fluorescence resonance energy transfer (FRET) results in the development of highly sensitive ratiometric fluorescence sensor probes with potential applications in bio‐imaging, metal sensing, membrane dynamics, and environmental sensing. In this review, the progress and recent developments in CDs based FRET systems utilized for various environmental applications are conferred. An in‐depth description is provided regarding the numerous donor/acceptor systems which when integrated with CDs generate efficient FRET systems. The review enables researchers to identify and develop specific systems which can be utilized to generate a FRET pair with potential physico–chemical properties that aid the development of the same for various applications.

Dual recognition ratio fluorescence-based sensor for sensitive detection of adenosine

The development of adenosine (AD) assays with high selectivity and sensitivity is of great importance for the early diagnosis of clinical diseases. In this study, we have engineered a straightforward, rapid, and highly sensitive ratiometric fluorescence sensor (BA-EuMOFs@MIP) capable of dual recognition of AD. The strategy employed here combines boronic acid-functionalized lanthanide metal–organic framework (BA-EuMOFs) with molecularly imprinted polymers (MIPs). In this scheme, BA-EuMOFs serve a dual purpose: they offer a stable and sensitive biometric fluorescence identification signal for BA-EuMOFs@MIP and induce changes in the fluorescence signal by identifying AD through the affinity of boric acid for cis-diol. Moreover, MIPs provide non-covalently imprinted recognition sites for AD recognition, giving the sensor satisfactory binding and selectivity to AD. Subsequently, we examined the principle of fluorescence changes, which can be attributed to fluorescence resonance energy transfer (FRET). More importantly, the BA-EuMOFs@MIP sensor demonstrates a wide linear range (1–50 mg/L) and sensitive detection limit (0.26 mg/L) for AD. Finally, we used the sensor to detect AD in real samples and obtained satisfactory recoveries (96.11–101.79%). This approach not only provides a simple, fast and sensitive sensing platform for AD detection but also shows great potential for early diagnosis of AD-related diseases.

A visible BODIPY-based sensor for ‘Naked-Eye’ recognition of Ag+ and its application on test paper strips

In this study, a novel, highly sensitive fluorescent sensor (E)-2-((2-(benzo[d] thiazol-2-yl) quinolin-8-yl) oxy)-N’-(4-(5, 5-difluoro-1, 3, 7, 9-tetramethyl-5H-4λ4, 5λ4-dipyrrolo [1, 2-c:2’, 1’-f] [1, 3, 2] diazaborinin-10-yl) benzylidene) acetohydrazide (TQB) was developed for dual mode of Ag+ detection (colorimetric/fluorescence), and its structural and spectral properties were characterized by 1H NMR, ESI-MS, X-ray, ultraviolet and fluorescence photometry. It is found that TQB could specifically and efficiently identify Ag+ among many other metal ions in CH3OH/H2O (7:3 v/v, pH = 7.23) buffer. The maximum absorption wavelength of TQB is red-shifted while its fluorescence is quenched with a quenching rate of 88.7%. The energy difference between TQB and TQB-Ag+ complex was calculated by DFT, and the applicability of TQB was verified by paper strip test. In addition, TQB has been successfully applied to the determination of Ag+ in real water samples with good reversibility and recoveries.

A water-stabilized Tb-MOF can be used as a sensitive and selective fluorescence sensor for the detection of oxytetracycline hydrochloride

Oxytetracycline hydrochloride (OTC) is a commonly used over-the-counter antibiotic, valued for its potent antibacterial properties. However, the inappropriate and excessive use of OTC can result in the accumulation of the drug in both the environment and human body, causing significant harm to ecosystems and human health. Therefore, the development of a fast and sensitive method for detecting OTC is of great significance. Lanthanide metal–organic frameworks (LnMOFs) can effectively excite lanthanide metals to emit long-lifetime, narrow and stable fluorescence based on the antenna effect, but their application in fluorescence sensing is rarely reported. In this work, a strongly fluorescent material Tb-MOF was synthesized by a facile solvothermal method using 1,2,4,5-phenylenetetracarboxylic acid (H4btec) and 1,10-phenanthroline (1,10-phen) as organic ligands and lanthanide metal Tb as the luminescent center. Tb-MOF is a stable material in water and shows excellent linearity with OTC in the concentration range of 0 ∼ 70 μM, with low detection limit (0.12 μM) and luminescence color transition from bright green to dark green during the detection process. X-ray diffraction, UV–vis absorption and fluorescence lifetime analyses revealed that the fluorescence quenching of Tb-MOF by OTC is caused by the inner filter effect in static quenching. Test strips for OTC detection were successfully prepared using Tb-MOF. These strips are not only low cost and easy to prepare but can also be used as portable sensing devices that can be easily distinguished by the naked eye during OTC testing. This study not only presents a fluorescent probe for the detection of OTC in water but also offers a practical method for converting fluorescent luminescent materials into functional devices for OTC detection.

Novel solution and solid-state emissive long-wavelength carbon dots for water sensing and white LED applications

Carbon dots (CDs) are among the most popular nanomaterials due to their remarkable fluorescent and electronic properties, as well as their good biocompatibility and low cytotoxicity. Currently, CDs with aggregation-induced emission (AIE) are relatively rare and have become a significant research hotspot. This is because most conventional CDs suffer from severe quenching in a solid state. Herein, novel CDs with both solution and solid-state emissions were obtained using a facile one-step hydrothermal synthesis. Specifically, the CDs exhibit yellow solvent-dependent fluorescence in solution state (λem = 580 nm) and red AIE emission in solid state (λem = 640 nm). CDs powder is utilized as a red phosphor for light-emitting diode (LED). The resulting fabricated white LEDs (WLEDs) demonstrate good performance metrics, including a Color Rendering Index (CRI) of 89.5, Correlated Color Temperature (CCT) of 3876 K, and commission Internationale d’Éclairage (CIE) coordinates of (0.350, 0.331). Furthermore, the solvent-dependent phenomenon observed in AIE-CDs can be exploited to apply their solution as a highly sensitive fluorescence sensor for quantitatively detecting amounts of water in various organic solvents. The method offers both high accuracy and sensitivity, with R2 values varying from 0.853 to 0.994 and a low detection limit ranging from 0.296 to 3.23 % across a wide linear range. This inherent versatility makes the CDs suitable for a wide range of applications, including sensing and LED devices.

Deep Eutectic Solvent-Based Highly Sensitive Turn-On Fluorescent Probe for D2O.

Owing to the importance of heavy water in spectroscopy, nuclear energy generation, chemical characterization, and biological industry, a design of a robust, cheap, nontoxic, and sensitive D2O sensor is very important. In this work, taking advantage of the singular emission fluorescence of the deep eutectic solvent prepared in our laboratory, we propose a first of its kind highly sensitive turn-on fluorescent sensor to effectively sense D2O at an ultratrace level based on rapid exchange of the labile DES proton with deuterium. This method can be used as a full-range heavy water detection strategy with a limit of detection of 0.079% (v/v) or 870 ppm. The isotopic purity (IP) obtained from DES fluorescence measurements is also in close agreement with that of the conventional FT-IR method. The current DES-based sensor thus allows both sensing and isotopic purity of D2O and can serve as one of the most sensitive monitoring strategies for heavy water analysis.

Highly sensitive and selective fluorescence and smartphone-based sensor for detection of L-dopa using nitrogen sulphur graphene quantum dots

The level of L-dopa in the body has a significant impact on treatment of Parkison’s disease. L-Dopa, a precursor to the neurotransmitter dopamine has been unanimously recommended as the gold standard for treating the condition. The present work focuses on creating a flexible platform for the quantitative and targeted detection of L-dopa using nitrogen sulphur co-dopped graphene quantum dots (NSGQD). The NSGQD were prepared by using thiourea and citric acid via calcination process. The synthesised NSGQD emitted bright bluish green light with the addition of L-Dopa to NSGQD in basic condition the fluorescence of NSGQD was quenched. L-dopa is transformed into dopaquinone in an alkaline environment and photoinduced electron transfer causes fluorescence quenching. As a result, the intensity of the NSGQD fluorescence gradually decreased as the L-dopa concentration rose in wide concentration range and low limit of detection (LOD) was established to be 7.8 nM relative to other analytes, quinone derivatives and amino acids the NSGQD showed greater specificity to L-dopa. The sensing mechanism was thoroughly investigated using steady state, time-resolved fluorescence spectroscopy, UV–Vis absorption measurements and fluorescent studies. Additionally, this also proves through smartphones portable device using color detector software. Our research paves the way for the creation of an affordable biosensor that will work well for finding L-dopa in actual samples. Computational studies were carried out using density functional theory for better understanding the interactions of dopa with NSGQD indicating hydrogen bonds are formed between carbonyl group of dopaquinone with functional groups of NSGQD.

Fe-MOF-based fluorescent sensor with on/off capabilities for the highly sensitive detection of tert-butylhydroquinone in edible oils

In this work, a “turn off-on” fluorescent sensor was developed for highly sensitive determination of tert-butylhydroquinone (TBHQ) based on an Fe(III)-based metal-organic framework (Fe-MOF). An Fe-MOF with an octahedral structure was synthesized via a simple hydrothermal method using ferric chloride hexahydrate and 2-aminoterephthalic acid (NH2-BDC) as raw materials. The fluorescence of Fe-MOF is extremely weak owing to ligand-to-metal charge transfer (LMCT) and internal filtration effect (IFE). When the system contained TBHQ, the binding of TBHQ to Fe(III) inhibited the LMCT of the fluorescent ligand NH2-BDC to Fe(III), releasing the fluorescence of NH2-BDC and thus restoring the fluorescence. With this as the basis, a rapid, sensitive, and selective fluorescence sensor is developed for the detection of TBHQ. Under the optimal conditions, TBHQ showed good linearity with fluorescence intensity in the range of 0–1.5 × 102 μmol L−1 and a detection limit of 0.0030 μmol L−1 (S/N = 3). The selectivity, reproducibility, and stability of the developed Fe-MOF-based sensors are comprehensively studied. Finally, the practicality of the method is verified by examining the detection of TBHQ in soybean oil; the results are consistent with those obtained using conventional high-performance liquid chromatography.

Dual-Responsive Carbon Quantum Dots for the Simultaneous Detection of Cytosine and 5-Methylcytosine Interpreted by a Machine Learning-Assisted Smartphone.

DNA methylation is an epigenetic alteration that results in 5-methylcytosine (5-mC) through the addition of a methyl group to the fifth carbon of a cytosine (C) residue. The methylation level, the ratio of 5-mC to C, in urine might be related to the whole-body epigenetic status and the occurrence of common cancers. To date, never before have any nanomaterials been developed to simultaneously determine C and 5-mC in urine samples. Herein, a dual-responsive fluorescent sensor for the urinary detection of C and 5-mC has been developed. This assay relied on changes in the optical properties of nitrogen-doped carbon quantum dots (CQDs) prepared by microwave-assisted pyrolysis. In the presence of C, the blue-shifted fluorescence intensity of the CQDs increased. However, fluorescence quenching was observed upon the addition of 5-mC. This was primarily due to photoinduced electron transfer as confirmed by the density functional theory calculation. In urine samples, our sensitive fluorescent sensor had detection limits for C and 5-mC of 43.4 and 74.4 μM, respectively, and achieved satisfactory recoveries ranging from 103.5 to 115.8%. The simultaneous detection of C and 5-mC leads to effective methylation level detection, achieving recoveries in the range of 104.6-109.5%. Besides, a machine learning-enabled smartphone was also developed, which can be effectively applied to the determination of methylation levels (0-100%). These results demonstrate a simple but very effective approach for detecting the methylation level in urine, which could have significant implications for predicting the clinical prognosis.

Non-aromatic single amino acid, lysine: A simple fluorescence sensor for selective detection of Cu2+ and Co2+ with colorimetric distinction by the virtue of its self-aggregation behaviour

Non-aromatic amino acids are mostly considered as non-emissive due to their lack of conventional fluorophores. However, at high concentrations, they can undergo conformational rigidification via self-aggregation and emit in the visible range. Herein, we have exploited that very property of non-aromatic amino acids as a fluorescence sensor. To the best of our knowledge, this is the first report demonstrating a single unmodified non-aromatic amino acid, Lysine (Lys) as a selective and sensitive fluorescence sensor for the detection of Cu2+ and Co2+ along with colorimetric distinction. An as-prepared aqueous solution of Lys (6.8 mM) served as a fluorescence sensor for Cu2+ and Co2+ with a detection limit of 420 nM and 460 nM, respectively in water. The instantaneous appearance of blue and pink color upon adding Cu2+ and Co2+, respectively to the Lys solution makes the system useful for their real-time analysis. The sensing of Cu2+ and Co2+ are also demonstrated through the fabrication of paper strips. Moreover, the sensor could detect Cu2+ and Co2+ in real water samples (lake water and tap water). The exceptionally low cost (<1 U.S. cent/10 mg Lys), reversibility of the sensor, and its ease of use uniquely qualify Lys as a remarkably simple fluorescent sensor.

A Sensitive Fluorescence Sensor for Tetracycline Determination Based on Adenine Thymine-Rich Single-Stranded DNA-Templated Copper Nanoclusters.

A sensitive fluorescent sensor has been developed for the determination of tetracycline (TC) using adenine thymine (AT)-rich single-stranded DNA (ssDNA) templated copper nanoclusters (CuNCs) as a fluorescent probe. Fluorescent ssDNA-CuNCs were synthesized by employing AT-rich ssDNA as templates and ascorbic acid as reducing agents through a facile one-step method. The as-prepared ssDNA-CuNCs exhibited strong fluorescence with a large Stokes shift (240 nm) and stable fluorescence emission. In the presence of TC, the fluorescent intensity of ssDNA-CuNCs was obviously decreased through the inner filter effect, due to the spectral overlapping between ssDNA-CuNCs and TC. Under the optimal conditions, the strategy exhibited sensitive detection of TC with a linear range from 2 nM to 30 μM and with a limit of detection of 0.5 nM. Furthermore, the sensor was successfully applied for the detection of TC in milk samples. Therefore, it provided a simple, rapid, and label-free fluorescent method for TC detection.

Boric acid-functionalized lanthanide metal-organic framework used as a ratiometric fluorescence probe for uric acid detection

Exploring high-performance fluorescent strategies for the highly sensitive determination of uric acid (UA) is of great importance for the early diagnosis of hyperuricemia. Herein, we constructed a simple and greatly sensitive ratiometric fluorescence sensor based on boric acid-functionalized lanthanide metal-organic framework (BA-Eu-MOF) for the determination of UA in complex environments. In this strategy, BA-Eu-MOF with dual-emission was first prepared by a one-step method at room temperature. It acts as a signal tag, allowing the sensor not only to exhibit a fast fluorescence response but also to generate an inverse response signal. This can be attributed to the inner filter effect, which enables amplification of the fluorescence signal and demonstrates satisfactory photostability, sensitivity and anti-interference capability. Benefiting from the efficient dual-channel strategy, BA-Eu-MOF sensor displayed a wide linear range (0–160 mg/L) for UA with a lower detection limit (0.06 mg/L). Moreover, the sensor has been successfully applied to the determination of UA in urine with satisfactory results. Accordingly, the developed ratiometric fluorescence sensors exhibit simple, fast, low-cost and highly sensitive properties that provide a reference for the detection of UA in complex clinical systems.

Environment friendly Co/Ag@ZnS quantum dots with high quality emission and application for selective sensing of Hg2+

Mercury (Hg), being a highly toxic environmental contaminant, poses serious ecological and health problems even at low quantities. Accurate and sensitive methods for detecting Hg2+ ions in environment are desperately needed. Also, the production of red emissive nanophosphors with outstanding color purity is a critical requirement for the next-generation display systems and lighting industry. A red emitting Co/Ag@ZnS QDs phosphor were prepared and studied to enhance the color characteristics of warm white light emitting diodes. The phosphors are synthesized using a co-precipitation technique, and the evolution of emission spectra are examined by the addition of dopants, significantly increasing PL QY up to 27.6%. A highly sensitive fluorescence sensor has been fabricated for Hg2+ detection using a quenching mechanism that has an excellent linear range of 0–50 µM, a correlation coefficient of 0.989, and a detection limit of 0.13 µM. More crucially, the proposed fluorescence sensing platform successfully and satisfactory detects Hg2+ in real water samples. These phosphors, which exhibit impressive CIE chromaticity coordinates, good CCT values, and high color purity (91.1%) may find substantial use in displays and demonstrate their effectiveness as red phosphors.

Facile preparation of a dual-emission paper-based fluorescent sensor based on carbon quantum dots and rhodamine B for dual-mode detection of Hg2+

In this study, carbon quantum dots (CQDs) and rhodamine B (RhB) were physically mixed to construct a highly selective and sensitive ratiometric fluorescent sensor (CQDs@RhB) for the quantitative and visual detection of Hg2+. After adding Hg2+, the emission peak of CQDs at 445 nm was quenched and the peak of RhB at 575 nm remained unchanged; the ratio of the two emission peaks had a linear relationship with the Hg2+ concentration. The CQDs@RhB had a detection range of 0–15 µM and the limit of detection (LOD) is 65.93 nM and a color change from blue to pink was visible to the naked eye under ultraviolet light. The study revealed a static quenching mechanism of CQDs@RhB. The established method was successfully applied to the detection of Hg2+ in real water and rice samples with satisfactory results. In addition, a portable fluorescent paper-based sensor platform was developed using computer software assistance. The relationship with Hg2+ concentration was constructed by converting the pictures obtained under ultraviolet light to RGB color mode for visualization and quantitative detection of Hg2+. This study provides a valuable strategy for constructing a rapid detection system for Hg2+ in the field.

Studied on highly sensitive fluorescent sensors for Fe3+, Nitrobenzene and dye adsorption properties of Ln-MOFs based on benzimidazole carboxylic acid ligand

Five new lanthanide metal-organic frameworks (Ln-MOFs) {[Ln(L)(H2O)5]·Cl}n (1–5), ([Ln=Pr(1), Nd(2), Eu(3), La(4), Ce(5)] based on benzimidazole carboxylic acid ligand [H2L = 2-(4-carboxyphenyl)-1H-benzo[d] imidazole-6-carboxylic acid]) with the same two-dimensional planar structure were synthesized by solvothermal method. Ln-MOFs 1–3 exhibited the good adsorption performance for the anionic dye Congo red (CR) and the adsorption capacities were 468, 449 and 438 mg·g−1, respectively. The adsorption kinetics indicated that the adsorption was chemical adsorption and the hydrogen bonds and π···π interactions between 1–3 hosts and CR guest molecules resulted in high adsorption capacities. Ln-MOFs 4 and 5 were developed for the exploration of fluorescence sensing performance. The results show that 4 and 5 can be used as fluorescent sensor for Fe3+ with good reusability, high sensitivity and selective detection, the detection limits were 1.1 × 10−5 and 4.3 × 10−6 M, respectively. Research shows that the quenching mechanism of Fe3+belongs to fluorescence resonance energy transfer (FRET). Meanwhile, 4 and 5 also have good detection effect on nitrobenzene (NB) and the detection limits were 1.6 × 10−6 and 2.4 × 10−7 M, respectively.

A highly sensitive hemicyanine-based near-infrared fluorescence sensor for detecting toxic amyloid aggregates in human serum

The development of an accurate and sensitive sensor for detecting amyloid plaques, which are responsible for many protein disorders like Alzheimer's disease, is crucial for early diagnosis. Recently, there has been a notable increase in the development of fluorescence probes that exhibit emission in the red region (>600 nm), aiming to effectively tackle the challenges encountered when working with complex biological matrices. In the current investigation, a hemicyanine-based probe, called LDS730, has been used for the sensing of amyloid fibrils, which belong to the Near-Infrared Fluorescence (NIRF) family of dyes. NIRF probes provide higher precision in detection, prevent photo-damage, and minimize the autofluorescence of biological specimens. The LDS730 sensor emits in the near-infrared region and shows a 110-fold increase in fluorescence turn-on emission when bound to insulin fibrils, making it a highly sensitive sensor. The sensor has an emission maximum of ~710 nm in a fibril-bound state, which shows a significant red shift along with a Stokes' shift of ~50 nm. The LDS730 sensor also displays excellent performance in the complicated human serum matrix, with a limit of detection (LOD) of 103 nM. Molecular docking calculations suggest that the most likely binding location of LDS730 in the fibrillar structure is the inner channels of amyloid fibrils along its long axis, and the sensor engages in several types of hydrophobic interactions with neighboring amino acid residues of the fibrillar structure. Overall, this new amyloid sensor has great potential for the early detection of amyloid plaques and for improving diagnostic accuracy.

A Turn-On Fluorescence Sensor Based on Nitrogen-Doped Carbon Dots and Cu2+ for Sensitively and Selectively Sensing Glyphosate

Glyphosate has excellent herbicidal activity, and its extensive use may induce residue in the environment and enter into humans living through the food chain, causing negative impact. Here, water-soluble 1.55 nm size nitrogen-doped carbon quantum dots (NCDs) with strong blue fluorescence were synthesized using sodium citrate and adenine. The maximum excitation and emission wavelengths of NCDs were 380 nm and 440 nm, respectively. The above synthesized NCDs were first used for the construction of a fluorescence sensor for glyphosate detection. It was found that Cu2+ could quench the fluorescence of NCDs effectively through the photoinduced electron transfer (PET) process, which was confirmed using fluorescence lifetime measurements. Additionally, the fluorescence was restored with the addition of glyphosate. Hence, a sensitive turn-on fluorescence sensor based on NCDs/Cu2+ for glyphosate analysis was developed. The LODs of glyphosate for water and rice samples were recorded as 0.021 μg/mL and 0.049 μg/mL, respectively. The sensor was applied successfully for ultrasensitive and selective detection of glyphosate in environmental water and rice samples with satisfied recoveries from 82.1% to 113.0% using a simple sample pretreatment technique. The proposed strategy can provide a significant potential for monitoring glyphosate residue in water and agricultural product samples.

Study of the pH effect on the optical and morphological properties of S, N self-doped carbon dots applied as fluorescent anti-counterfeiting ink and pH sensor

The pH value is an important parameter as it is part of several processes, whether environmental or biological. In this report, S, N self-doped carbon dots (CDs) were synthesized by a simple hydrothermal method using cysteine (cys) and citric acid as precursors for a detailed investigation of size, morphological, photoluminescent, and structural changes at different pH values and its use as pH sensor and fluorescent ink. The fluorescence intensity of cys-CDs was dependent on the pH, presenting a linear relationship with pH values in the range of 2.0–9.0. Using spectroscopic techniques, a mechanism for the pH-dependent fluorescence is proposed, based on the aggregation of cys-CDs and also protonation/deprotonation of surface functional groups that change the excited state. The cys-CDs were found to be efficient as fluorescent pH sensors using real samples (distilled water and tap water). Furthermore, the pH changes in cys-CDs can be used for the visual enhancement of anti-counterfeiting technologies. Thus, the results of this study show that cys-CDs can act as an efficient and pH sensitive fluorescent sensor, which can be used to measure the pH value of water samples, due to its high fluorescence intensity, and can be applied successfully as a fluorescent ink.