Synthesis of nitrogen- and iron-containing carbon dots, and their application to colorimetric and fluorometric determination of dopamine

Multifunctional N, Fe-doped carbon dots with peroxidase-like activity for the determination of H2O2 and ascorbic acid and cell protection against oxidation

Multifunctional N, Fe-doped carbon dots (N, Fe-CDs) were synthesized by the one-step hydrothermal method using ferric ammonium citrate and dicyandiamide as raw materials. The N, Fe-CDs exhibited peroxidase-like (POD) activity by catalyzing the oxidization of 3,3',5,5'-tetramethylbenzidine (TMB) to the green oxidation state ox-TMB in the presence of hydrogen peroxide (H2O2). Subsequently, based on the POD activity of N, Fe-CDs, an efficient and sensitive colorimetric method for the detection of H2O2 and ascorbic acid (AA) was established with a limit of detection of 0.40 µM and 2.05 µM. The proposed detection method has been successfully applied to detect AA in fruit juice, vitamin C tablets, and human serum samples and has exhibited excellent application prospects in biotechnology and food fields. Furthermore, N, Fe-CDs also showed a protective effect on the cell damage caused by H2O2 and could be used as an antioxidant agent.

The Guidelines for the Design and Synthesis of Transition Metal Atom Doped Carbon Dots

Atoms doping is a practical approach to modulate the physicochemical properties of carbon dots (CDs) and thus has garnered increasing attention in recent years. Compared to non-metal atoms, transition metal atoms (TMAs) possess more unoccupied orbitals and larger atomic radii. TMAs doping can significantly alter the electronic structure of CDs and bestow them with new intrinsic characteristics. TMAs-doped CDs have exhibited widespread application potential as a new class of single-atom-based nanomaterials. However, challenges remain for the successful preparation and precise design of TMAs-doped CDs. The key to successfully preparing TMA-doped CDs lies in anchoring TMAs to the carbon precursors before the reaction. Herein, taking the formation mechanism of TMAs-doped CDs as a starting point, we systematically summarized the ligands employed for synthesizing TMAs-doped CDs and proposed the synthetic strategy involving multiple ligands. Additionally, we summarize the functional properties imparted to CDs by different TMA dopants to guide the design of TMA-doped CDs with different functional characteristics. Finally, we describe the bottlenecks TMAs-doped CDs face and provide an outlook on their future development.

Fe-codoped carbon dots serving as a peroxidase mimic to generate in situ hydrogen peroxide for the visual detection of glucose

Nanozyme technology has gained significant regard and been successfully implemented in various applications including chemical sensing, bio-medicine, and environmental monitoring. Fe-CDs were synthesized and characterized well in this study. As compared to HRP (3.7 mM), the Fe-CDs exhibited a higher affinity towards H2O2 (0.2 mM) using the steady-state kinetic assay and stronger catalytic capability by changing the color of TMB to the blue color of the oxidized state, oxTMB. Additionally, an efficient peroxidase mimic, Fe-CDs/GOx, based on the hybrid cascade system to produce in situ H2O2 for the visual detection of glucose (color change: colorless to blue, and then to green), has been developed in detail, with limits of detection (LODs) for H2O2 and glucose of 0.33 μM and 1.17 μM, respectively. The changes further demonstrate a linear relationship between absorbance and H2O2 concentration, ranging from 10 to 60 μM, and for glucose (1 to 60 μM). To assess the accuracy and detection capability of the Fe-CDs/GOx system, we evaluated a real human serum sample obtained from adult males in a local hospital. In conclusion, Fe-CDs serving as a peroxidase mimic have the potential for various applications in the fields of biomedicine and nanozymes.

A new Fe/N doped carbon dot naocatalytic amplification-aptamer SERS/RRS/Abs trimode assay platform for ultratrace Pb2

Nanocatalytic amplification of carbon dots is a new way to improve sensitivity. The preparation of the high catalytic activity and stable iron/nitrogen-doped carbon dot (CD_BFe) sol and its application in aptamer (Apt) assay have not been reported yet. In this paper, a simple hydrothermal procedure for the preparation of CD_BFe derived from Fe²⁺-2,2'-bipyridine complex has been developed. It is found that CD_BFe has a strong catalytic effect on the indicator reaction of glyoxal (C₂H₂O₂) reduction of HAuCl₄ to produce gold nanoparticle (AuNP) probe with strong surface enhanced Raman scattering (SERS) at 1617 cm^-1, resonance Rayleigh scattering (RRS) effect at 370 nm and absorption (Abs) at 550 nm. A rapid and sensitive CD_BFe catalytic amplification Apt method for SERS/RRS/Abs trimode detecting ultratrace lead ions was established, based on the Apt reaction mediated the nanocatalytic indicator reaction. The results show that the SERS intensity and Pb(II) concentration have a good linear relationship in the range of 1.3-16 pM, and the detection limit is the lowest. In addition, Hg(II) and As(III) can also be measured by this naocatalytic amplification- Apt assay platform.

Z-scan analysis and theoretical studies of dopamine under physiological conditions

Dopamine (DA) is a widely researched catecholamine best known for its role in motor, motivation, addiction, and reward. Disruption in dopamine homeostasis and signaling within the central nervous system (CNS) can lead to disorders such as attention deficit hyperactivity disorder (ADHD), schizophrenia, Parkinson's disease, and obsessive-compulsive disorder. In the periphery, circulating DA is stored in blood platelets, and its disruption correlates with pathological conditions such as head and neck paragangliomas, Huntington's chorea, and schizophrenia. Various methods to sensitively and selectively detect dopamine have been reported, but sparse attempts have been made to exploit its intrinsic properties. Previously, we have harnessed dopamine's natural mid-ultraviolet auto-fluorescence to carry out its label-free imaging in live brain tissues. Recently, we used the closed-aperture (CA) Z-scan method to provide the first line of evidence on the existence of dopamine nonlinearity. Here, we utilized this simple, sensitive, and straightforward CA Z-scan technique and coupled this with theoretical simulations to further investigate the nonlinear photophysical properties of DA under physiological conditions. Our combined approach revealed that the nonlinear property of dopamine is governed by the thermo-optical effects, and the CA Z-scan profiles can be modulated by parameters such as phase-shift, orders of absorption, and time dependency. Simple and physiologically relevant systems, such as the platelets, are amenable to Z-scan analysis, thereby empowering us to scrutinize in the future if nonlinearity and its alterations, if any, have a direct bearing on DA homeostasis and associated diseases.

Green and facile synthesis of iron-doped biomass carbon dots as a dual-signal colorimetric and fluorometric probe for the detection of ascorbic acid

A new method based on biomass Fe-CDs with fluorescence properties and simulated oxidase activity colorimetric and fluorometric dual-readout assay for highly effective detection of AA was established.

Nitrogen-Doped Carbon Dot and CdTe Quantum Dot Dual-Color Multifunctional Fluorescent Sensing Platform: Sensing Behavior and Glucose and pH Detection

A novel fluorescent probe based on a nitrogen-doped carbon dot (N-CD) and CdTe quantum dot (CdTe QD) platform has been constructed for H₂O₂/glucose detection and pH sensing. In this work, H₂O₂-tolerant blue fluorescence N-CDs were added to the H₂O₂-mediated yellow fluorescence quenching of CdTe QDs to construct a dual-color ratiometric fluorescent H₂O₂ probe. H₂O₂-induced passivated group detachment and action on deep nanocrystals promoted CdTe QD fluorescence quenching. Meanwhile, the addition of the blue fluorescent background of N-CDs sharply reflected the color change in CdTe QDs. Under the optimized experimental conditions, the platform was effectively applied to the detection of H₂O₂ produced by the enzymatic reaction of glucose, showing high sensitivity (limit of detection 7.86 μM) and wide linear range (26-900 μM) for glucose detection. The pH-sensing behavior of CdTe QDs and N-CDs was attributed to the displacement of a weak acid (3-mercaptopropionic acid) by a strong acid (HCl) and the acid titration process of two coexisting bases (N-CDs and NH₃·H₂O), respectively. The loss of passivation and doping effects led to a decrease in the fluorescence intensity of CdTe QDs and N-CDs. Moreover, utilizing the ability of bimaterial system fluorescence to pH sensing, a semiquantitative pH detection based on the linear response was developed. The pH range was analyzed by three kinds of N-CD (F_ex = 440 nm) and CdTe QD (F_ex = 548 nm) typical emission spectral shapes. In addition, the recovery results showed that the bimaterial system was proved to be appropriate for the assay of glucose in spiked serum samples.

Nitrogen doped graphene quantum dots based on host guest interaction for selective dual readout of dopamine

Herein, yellow emissive nitrogen doped graphene quantum dots (N@GQDs) were prepared by a novel advanced thermal driven oxidation. The N@GQDs was functionalized with β-cyclodextrin (β-CD) to improve its catalytic performance towards dopamine (DA) detection. The β-CD/N@GQDs exhibited strong fluorescence at λ_em. = 550 nm after excitation at 460 nm with a quantum yield of 38.6%. The β-CD/N@GQDs showed good peroxidase like activity via catalyzing the oxidation of tetramethylbenzidine (TMB) in presence of H₂O₂ to form blue colored product at λ_max = 652 nm. In the colorimetric assay of DA, the detection based on the oxidation of TMB by H₂O₂ in presence of β-CD/N@GQDs as a catalyst. Then, the color of the blue oxidized TMB (oxTMB) product was reduced by addition of DA. While the fluorometric detection of DA based on the "inner filter effect" of the overlapped emission spectrum of β-CD/N@GQDs with the absorption spectrum of oxTMB, where, addition of DA reduces oxTMB to TMB and restores the fluorescence intensity of β-CD/N@GQDs. Under the optimized conditions, the colorimetric method achieved linearity range of 0.12-7.5 µM and LOD (S/N = 3) of 0.04 µM, while the fluorometric method achieved linearity range of 0.028-1.5 µM and LOD (S/N = 3) of 0.009 µM. The peroxidase like activity of β-CD/N@GQDs was used to detect DA in human plasma and serum samples with good % recoveries. The colorimetric and fluorometric methods exhibited good sensitivity and selectivity toward DA detection.

Aptamer Turn-On SERS/RRS/Fluorescence Tri-mode Platform for Ultra-trace Urea Determination Using Fe/N-Doped Carbon Dots

Sensitive and selective methods for the determination of urea in samples such as dairy products are important for quality control and health applications. Using ammonium ferric citrate as a precursor, Fe/N-codoped carbon dots (CD_FeN) were prepared by a hydrothermal procedure and characterized in detail. CD_FeN strongly catalyzes the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H₂O₂ to turn on an indicator molecular reaction, forming an oxidized tetramethylbenzidine (TMB_ox) probe with surface-enhanced Raman scattering, resonance Rayleigh scattering, and fluorescence (SERS, RRS, and FL) signals at 1,598 cm⁻¹, 370 nm, and 405 nm, respectively. The urea aptamer (Apt) can turn off the indicator reaction to reduce the tri-signals, and the addition of urea turns on the indicator reaction to linearly enhance the SERS/RRS/FL intensity. Thus, a novel Apt turn-on tri-mode method was developed for the assay determination of ultra-trace urea with high sensitivity, good selectivity, and accuracy. Trace adenosine triphosphate and estradiol can also be determined by the Apt-CD_FeN catalytic analytical platform.

Design of "Turn On" fluorometric nanoprobe based on nitrogen doped graphene quantum dots modified with β-cyclodextrin and vitamin B6 cofactor for selective sensing of dopamine in human serum

Herein, a novel and rapid fluorometric nanoprobe was constructed for quantitation of dopamine (DA) in presence of biologically interfering compounds. The nanoprobe based on synthesis of yellow emissive nitrogen doped graphene quantum dots (N@GQDs) by advanced thermal driven oxidation. After that, the synthesized N@GQDs was capped with β-cyclodextrin (β-CD), followed by interaction with pyridoxal (PYL) vitamin B₆ cofactor. This interaction resulted in diminishing the yellow fluorescence of β-CD/N@GQDs, and appearance of blue emission peak at 420 nm. Upon addition of DA, the blue emission of β-CD/N@GQDs was increased after excitation at λ = 330 nm. Under optimum conditions, the nanoprobe exhibited a linear range of 0.36-400 nM with limit of detection (LOD) of 0.117 nM. In addition, the fluorescent nanoprobe shows high selectivity and can be used for detection of DA in complicated biological matrices and human serum. This strategy might provide a potential tool for clinical diagnosis and biomedical research for DA related diseases.

Selective Determination of Dopamine in Pharmaceuticals and Human Urine Using Carbon Quantum Dots as a Fluorescent Probe

A cost-effective and environmentally friendly method was formulated for rapid dopamine (DA) detection that was based on the fluorescence (FL) quenching of carbon quantum dots (C-dots). Upon adding DA to the C-dots’ solution, we noticed a regular reduction in their fluorescence intensity. The effects of pH, amount of C-dots, reaction temperature and time on the determination of DA were investigated. Under the optimized experimental conditions, trace amounts of DA could be analyzed. Furthermore, dopamine hydrochloride injection and human urine samples with and without spiked DA were analyzed using the developed sensing system. The procedure was validated following the guidelines of the European Medicines Agency (EMA) in terms of the following: calibration range (0.3–100 μM), linearity (R2 = 0.9991), limit of detection (LOD) (93 nM). Recoveries of dopamine with spiked samples at three different levels were between 95.0 and 105.9%, and the relative standard deviations (RSDs) were within 2.68% (n = 6). This method is simple and suitable for the determination of dopamine in pharmaceuticals and human urine for clinical application. Compared with previous reports, the proposed method offers great advantages including ease of C-dot sensor preparation (one-pot synthesis), environmentally friendly sample preparation by using either water or phosphate buffer solution only, a short response time and selectivity.

Carbon dots as naked eye sensors

Optical sensors are always fascinating for chemists due to their selectivity, sensitivity, robustness and cost-effective nature.

Boron-doped carbon quantum dots: a 'turn-off' fluorescent probe for dopamine detection

Boron-doped carbon quantum dots (size 2.3 nm) were fabricated by a modified hydrothermal carbonization one-pot synthesis protocol using 4-hydroxy phenylboronic acid as the common precursor that provided seed for the formation of carbon quantum dots as well as the dopant. These quantum dots exhibited excellent properties, namely good aqueous dispersion, strong fluorescence emission, good environmental stability, high selectivity and sensitivity towards the neurochemical dopamine even in the absence of any linker, functionalizing agents or enzyme. It is shown that this material can be used as a 'turn-off' fluorescent probe for the detection of even low concentrations of dopamine with a limit of detection (3σ/S) of about 6 μM. The simplicity of the synthesis protocol and the ease of dopamine detection define the novelty of this approach.

Smartphone-Based Electrochemical Potentiostat Detection System Using PEDOT: PSS/Chitosan/Graphene Modified Screen-Printed Electrodes for Dopamine Detection

In this work, a smartphone-based electrochemical detection system was designed and developed for rapid and real-time detection of dopamine (DA). The system included a screen-printed electrode (SPE) used as a sensor, a hand-held electrochemical potentiostat and a smart phone with a specially designed app. During the detection period, the SPEs modified with poly(3,4-ethylenedioxythiophene) (PEDOT), chitosan (CS) and graphene (G) were used to convert and amplify the electrochemical reaction signals. The electrochemical potentiostat was used to generate excitation electrical signals and collect the electrical signals converted from the sensor. The smartphone—connected to the detector via Bluetooth-was used to control the detector for tests, further process the uploaded data, and plot graphs in real time. Experimental results showed that the self-designed sensing system could be employed for highly selective detection of DA in the presence of interfering substances such as ascorbic acid (AA) and uric acid (UA). CV was carried out to characterize the electrochemical properties of the modified SPEs and the electrochemical behaviors of DA on the modified SPEs. Finally, according to the analysis of DPV responses of DA, the system could detect DA with a detection sensitivity of 0.52 ± 0.01 μA/μM and a limit of detection of 0.29 μM in the linear range of DA concentrations from 0.05 to 70 μM.

Recent Developments in Synthesis and Photocatalytic Applications of Carbon Dots

The tunable photoluminescent and photocatalytic properties of carbon dots (CDs) via chemical surface modification have drawn increased attention to this emerging class of carbon nanomaterials. Herein, we summarize the advances in CD synthesis and modification, with a focus on surface functionalization, element doping, passivation, and nanocomposite formation with metal oxides, transition metal chalcogenides, or graphitic carbon nitrides. The effects of CD size and functionalization on photocatalytic properties are discussed, along with the photocatalytic applications of CDs in energy conversion, water splitting, hydrogen evolution, water treatment, and chemical degradation. In particular, the enzyme-mimetic and photodynamic applications of CDs for bio-related uses are thoroughly reviewed.

Organosilane-functionalized carbon quantum dots and their applications to "on-off-on" fluorometric determination of chromate and ascorbic acid, and in white light-emitting devices

Organosilane-functionalized carbon quantum dots (Si-CQDs) were synthesized by reacting glucosamine and 3-[2-(2-aminoethylamino)ethylamino]propyl-trimethoxysilane in acetone. The surface morphology, crystal structure, functional groups, elemental composition, and optical properties of the Si-CQDs were characterized using TEM (HRTEM), XRD, FT-IR, XPS, UV-vis absorption and fluorescence spectroscopy. They show that N-containing groups including C=N and C-N, and Si-containing groups including Si-O-C and Si-O-Si have been formed on the surface of Si-CQDs. The element doping and surface functionalization of Si-CQDs endow their novel chemical, physical and optical properties. The Si-CQDs dispersed in acetone are almost monodisperse with an average particle diameter of 3.6 nm. The Si-CQDs dispersed in acetone display blue fluorescence (excitation/emission maxima of 380/460 nm). In contrast, the solid-state Si-CQDs exhibited yellow fluorescence (with excitation/emission maxima of 470/595 nm). The fluorescence emission spectra of acetone-suspended Si-CQDs are concentration-dependent, and the emission peak becomes red-shifted as the concentration is increased. The Si-CQDs are sensitive and selective fluorescent "on off on" nanoprobes for chromate [Cr(VI)] and ascorbic acid (AA). Fluorescence is quenched by Cr(VI) via an inner filter effect from the absorption of Si-CQDs excitation at 380 nm by Cr(VI). Upon addition of AA, fluorescence is restored because of reduction of Cr(VI) by AA. Under optimal conditions (excitation/emission wavelength of 380/460 nm), the response is linear in the 0.4-160 μM Cr(VI) concentration range, and the detection limit is 34 nM. The respective data for AA are 1-80 μM and 84.6 nM. The practical use of the nanoprobe for Cr(VI) determination in real river water samples is also demonstrated successfully. Their concentration-dependent fluorescence, good thermal stability and self-crosslinking behavior also make the Si-CQDs a candidate material for white light-emitting diodes that displays color conversion and can act as an encapsulation layer in a blue light-emitting diode (LED) chip. Graphical abstract One-pot solvothermal synthesis of organosilane-functionalized carbon quantum dots (Si-CQDs) with blue fluorescence in solution, yellow fluorescence in solid state and concentration-dependent fluorescence property, and their applications for chromate (Cr(VI)) and ascorbic acid dual determinations and white light-emitting device. Graphical Abstract contains poor quality and small text inside the artwork. Please do not re-use the file that we have rejected or attempt to increase its resolution and re-save. It is originally poor, therefore, increasing the resolution will not solve the quality problem. We suggest that you provide us the original format. We prefer replacement figures containing vector/editable objects rather than embedded images. Preferred file formats are eps, ai, tiff and pdf.We have changed the poor quality graphical abstract into the jpg and pdf.

Facile fabrication of fluorescent Fe-doped carbon quantum dots for dopamine sensing and bioimaging application

A facile Fe-doped carbon quantum dot based fluorescent sensor for dopamine sensing and bioimaging was constructed.

Carbon Dot Nanozymes: How to Be Close to Natural Enzymes

The design, catalytic process, and property study of nanozymes are of importance for both fundamental research and application demand. Here, the peroxidase-mimicking properties of a series of carbon dots (C-dots) was systematically investigated and they were found to be probably closer to their natural counterparts, as compared to the known corresponding nanozymes. Firstly, four kinds of metal-free and surface-modulated C-dots were bottom-up fabricated using glucose, α-cyclodextrin (CD), β-CD, and γ-CD as precursors, respectively, and their formation processes, structures, as well as surface chemistry were investigated. Secondly, in the peroxidase-mimicking catalytic system, no hydroxyl radicals were produced, which indicates a different and special catalytic mode. By employing a joint experimental-theoretical study, a probable catalytic mechanism is proposed. Thirdly, the present C-dots maintained well their catalytic activity even in complicated serum matrices because their catalytic performances are completely irrelevant of any cation-related binding sites. Finally, the catalytic performances of the as-prepared C-dots were modulated by either pre-engineering NP surface structures or subsequently introducing photo-regulated host-guest reactions.

A specific fluorescent nanoprobe for dopamine based on the synergistic action of citrate and gold nanoparticles on Tb(III) luminescence

A nanoprobe was developed for the fluorometric determination of the neurotransmitter dopamine (DA). It is based on the synergistic enhancement action of citrate and gold nanoparticles (AuNPs) on the luminescence of Tb(III). AuNPs serve as substrates of surface enhanced fluorescence (SEF). Citrate, in turn, acts as a spacer for the SEF effect, a co-ligand of Tb(III) complex, and a recognizing component for DA. The synergistic action of citrate and AuNPs significantly increases the intrinsic green fluorescence of Tb(III) (best measured at excitation/emission peaks of 300/547 nm). Under the optimum conditions, the fluorescence intensity increases linearly in the 3.0 to 200 nM DA concentration ranging (with an R² value of 0.9959), and the limit of detection (at S/N = 3) is 0.84 nM. The nanoprobe shows good selectivity for DA among other interfering neurotransmitters, some amino acids and ions. The method was applied to the detection of DA in human serum samples where it gave recoveries ranging from 100.5 to 102.9%. Graphical abstract Schematic of a Tb(III) composite fluorescent nanoprobe for the sensitive determination of dopamine (DA). Citrate and gold nanoparticles (AuNPs) synergistically enhance the fluorescence of Tb(III)-DA.

One-pot synthesis of highly fluorescent Fe2+-doped carbon dots for a dual-emissive nanohybrid for the detection of zinc ions and histidine

Fe²⁺ was confirmed to be the only definitive one of the common metal ions to synthesize the highly fluorescent carbon dots with proline as the carbon resource at 80 °C for visual fluorescence sensing Zn2+ and histidine, respectively.

Fluorimetric and colorimetric analysis of total iron ions in blood or tap water using nitrogen-doped carbon dots with tunable fluorescence

Nitrogen-doped Cdots were fabricated with tunable blue-green fluorescence and changing of color for fluorimetric and colorimetric assays for total iron.

A fluorometric and colorimetric dual-mode sensor based on nitrogen and iron co-doped graphene quantum dots for detection of ferric ions in biological fluids and cellular imaging

A dual-mode sensing strategy based on N, Fe-GQDs for effective and selective detecting of Fe³⁺ and cellular imaging was developed.

Inner filter effect-based fluorescent sensing systems: A review

Inner filter effect (IFE) was previously considered as an error in fluorescence measurement. In recent years, it has been developed as an important non-irradiation energy conversion model of spectroscopic technique and found wide applications in the fields of chemical sensing and biosensing. In comparison with traditional techniques based on forster resonance energy transfer (FRET), the IFE-based fluorescent approach is more flexible and straightforward without the link of absorber with fluorescer. The present review for the first time introduces the state of the art in the progress of the IFE-based fluorescent sensing systems, including sensing strategy, essential conditions, materials option, and their applications for the detection of various target analytes, e.g., ionic species, small molecules, and macromolecules. In addition, the benefits and limitations of the IFE-based fluorescent sensing systems are also critically discussed and highlighted.