pH-dependent Photoluminescence Research Articles

Study on pH dependent ON/OFF fluorescent switching behaviour of hydrothermally synthesised vanadium disulphide quantum dots (VS2-QDs)

It is fascinating to design and develop an inorganic quantum dot (QDs) which exhibit luminescence features with ion concentration, temperature, and pH sensitivity dependence. Vanadium disulphide (VS2) belongs to the family of transition metal dichalcogenides (TMDs), and is extensively studied owing to its layered characteristics features in sensing and filtrations technology, however, its fluorescence features have not been well studied so far. The current work describes a widely adopted eco-friendly hydrothermal approach for the synthesis of VS2-QDs. The average size of particles was determined to be ∼10 nm by transmission electron microscopy (TEM) and Zetasizer analysis. The synthetic approach adopted here facilitates in situ functionalization of QDs, which results in their high stability in aqueous medium and enhanced sensitivity towards their surroundings. As most of the QDs show the sensitivity towards pH, a thorough investigation of VS2 QDs for their pH dependent photoluminescence has been carried out. The VS2-QDs were discovered to exhibit a remarkable luminescence intensity that was found to be about 11 times higher in basic conditions (pH ∼ 13, QY = ∼4.80 %) as compared to that in acidic conditions (pH ∼ 1, QY = ∼1.10 %). The root cause of this ON/OFF florescence flipping may be related to the functional groups (such as −SO42−, −NH2, OH−, etc.) attached over the surface of QDs, acid etching, and protonation–deprotonation process. This paper highlights the pH dependent switching behaviour of VS2-QDs that could offer valuable information for designing a futuristic pH-based device for biomedical applications.

Regulating optoelectronics of carbon dots with redox-active dopamine

The electron transfer (ET) processes in carbon-based quantum dots (CDs) have laid to extensive research endeavors due to their tunable optoelectronic properties with alteration in surface functionalization, doping, surface charge, etc. Here, we proffer shreds of evidence on pH-dependent electron transfer and transport properties of undoped CDs (UCDs) and amine-doped CDs (ACDs) with a redox-active neurotransmitter, dopamine (DA). A pronounced pH-dependent photoluminescence (PL) quenching is depicted for both the CDs with DA, where the quenching efficiency substantially increases in alkaline solvents. Taking advantage of the structural transformation of DA from hydroquinone to quinone, the electron accepting capacity can be improved for alkaline pH, which enhances the ET efficiency. The pH-regulated conductance measurements across the metal-CD-metal junction reveal a hike in the conductivity for acidic and alkaline pH relative to the neutral one. The I-V traces of UCDs and ACDs are contributed by an initial linear current rise and non-linear growth at higher bias explicating both direct and Fowler-Nordheim (F-N) tunneling mechanisms. In addition, the incorporation of DA into these CDs, not only significantly increases conductance in all pH media but also enables them to tunnel the potential barrier directly in acidic media, reducing F-N tunneling. These results could provide new avenues for developing reliable optoelectronic devices and sensing probes for a wide range of applications.

Cu-based metal-organic frameworks-derived copper nanoclusters with tunable emission for ratiometric pH sensing

Copper nanoclusters (Cu NCs) have emerged as promising platform for biosensing, but it remains a challenge to fabricate highly stable Cu NCs with controllable photoluminescence. Herein, we propose a new and elaborate strategy to prepare photoluminescent Cu NCs by using Cu-based metal-organic frameworks (Cu-MOFs) as the precursor. The generated Cu NCs display ordered nanowire structure and superior stability due to the crystal structure of Cu-MOFs. Moreover, the Cu NCs possess unique pH-dependent photoluminescence property. With pH increasing, the yellow-green emission of Cu NCs at 520 nm decreases, and new cyan emission at 455 nm appears and increases gradually. This process might be related to the structure change of the Cu NCs from aggregated nanowire into dispersed spherical particle. On the basis, a facile ratiometric fluorescent sensing platform for pH is fabricated. This ratiometric sensor has great anti-interference capacity and excellent reversibility. In addition, it was successfully applied to monitor the pH of microorganisms, demonstrating the great potential in bioassays. This new synthetic strategy for preparing Cu NCs with tunable emission property may propose new opportunities for their applications.

A sensitive chemosensor for nitro-containing compounds based on Au nanoclusters/Ba2+ co-assembly system: The crucial role of ligands to metal charge transfer

Gold nanoclusters (AuNCs) are a class of promising inorganic-organic hybrid nanomaterials with exceptional photoluminescent properties. The understanding of their photoluminescent mechanism is important for developing functional AuNCs-based materials. In this work, thiobarbituric acid-modified AuNCs (TBA-AuNCs) was prepared and its photoluminescent property was modulated through co-assembly with Ba2+. The emission intensity was sharply enhanced with the formation of TBA-AuNCs/Ba2+ complex. Structural characterization including transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy confirmed the hybridization of TBA-AuNCs with Ba2+. The pH-dependent photoluminescence as well as zeta potential measurements demonstrated that the ligand to metal charge transfer (LMCT) is crucial for the regulation of emission property of the TBA-AuNCs/Ba2+. Based on this special photoluminescent mechanism, the TBA-AuNCs/Ba2+ complex was employed as a chemical sensor for nitro-substituted aromatic compounds. The emission intensity of TBA-AuNCs/Ba2+ selectively displayed sharp quenching in response to 2-methyl-4-nitroimidazole among various imidazole derivatives. Simultaneously, the complex also showed high sensitivity toward p-nitroaniline and 2-nitrophenol. Therefore, the developed TBA-AuNCs/Ba2+ assemblies are potential in detecting the nitro-containing compounds, which will find applications in pharmaceutical analysis, environmental protection and other related fields.

Red/Green Tunable-Emission Carbon Nanodots for Smart Visual Precision pH Sensing

The development of novel carbon nanodots (CDs) with excellent optical properties and promising applications is an attractive but a challenging area. Herein, we demonstrate that the fluorescence properties of CDs can be optimized by simple modification of the molecular structure of the carbon source. Significantly, the final optimized CDs 1 exhibit a rare extremely acidic (pH 1-3) sensitivity. A distinguishable red/brown/green fluorescence color change is achieved for visual readout by simply tuning the pH to 1, 2, or 3. The distinguishable color change combined with ratiometric fluorescence changes greatly improves the naked-eye visual resolution up to 0.2 pH units, which is higher than that of commercial precision pH test strips. Furthermore, a smartphone was employed in a portable, low-cost, rapid, and precise pH measurement method, which satisfied the requirement of point-of-care testing. Inspired by the reversible color change of red/brown/green tri-fluorescence on tuning the pH to 1, 2, or 3, a simple yet efficient molecular logic circuit has been designed. Given the unique pH-dependent photoluminescence properties, these CDs have been further applied in the anti-counterfeiting and information encryption field, making the burn after readingencryption technology a reality.

Understanding the pH dependent fluorescence and doxorubicin release from graphene oxide functionalized citric acid dendrimer as a highly efficient drug delivery system

In this work, the third generation of a dendrimer based on citric acid and 1, 3, 5 benzene tricarbonyl trichloride, as a core (G3), was synthesized to modify graphene oxide (GO) surface. A novel graphene oxide hybrid (GO-G3) with green fluorescent in an acidic medium (pH 4) was developed to track its interaction with cancer and healthy cell lines as a potential drug carrier. GO-G3 is highly fluorescent due to hydrogen bonding that passivates the electron-hole recombination process. The interaction among G3 dendrimer and GO surface was investigated at two pH in detail as the main cause for observation of such pH-dependent fluorescent in acidic media. The functionalized GO were studied carefully and characterized by multiple techniques such as fluorescence spectroscopy; SEM (scanning electron microscope); TEM (Transmission electron microscopy), FT-IR (Fourier transformed infrared spectroscopy); XRD (X-ray diffraction spectroscopy); XPS (X-ray photoelectron spectroscopy), DLS (Dynamic light scattering), AFM (atomic force microscopy); UV-Vis absorption spectroscopy, Raman spectroscopy and CLSM (Confocal laser scanning microscopy). The synthesized dendrimer displayed no emission under a UV lamp. However, after the functionalization of the GO surface with G3 dendrimer, green photoluminescence was detected at 471 nm. G3 dendrimer covers GO surface and prevents both GO sheets restacking and also electron-hole recombination process that consequently led to the observation of pH-dependent photoluminescence (PL). The drug encapsulation capacity of hybrid for doxorubicin (DOX) drug was 159.48% and also, the drug release profile of GO-G3/DOX hybrid indicated a pH-dependent release with more preference for drug discharge in acidic media. Furthermore, the cytotoxicity investigation of GO-G3/DOX against T47D cells (human breast cancer cells) indicated the IC50 value of 12.02 μg/ml.

Zinc Magnesium Oxide-Based Nanorods for High-Precision pH Sensing

We developed a zinc magnesium oxide–based nanosensor platform for pH sensing. pH is a parameter strongly influential in various chemical, biological, and biomedical applications. Oxide-based semiconductors are preferred as pH sensing materials because of their robustness, size modulation, high sensitivity, cost effectiveness, reversibility of the sensing process, and long lifetime. Vertical nanorods of zinc magnesium oxide were fabricated for the nanosensor platform. Their high crystalline quality was confirmed through various structural characterization techniques such as high-resolution X-ray diffraction and scanning and transmission electron microscopy. Absorption spectroscopy revealed a bandgap of 3.56 eV corresponding to 12% magnesium. pH dependent photoluminescence (PL) measurements showed a shift in the visible range spectrum with decrease in PL intensity. The developed sensor showed high sensitivity of 44.4 ± 3.8 mV/pH, close to the theoretical value, in the 2.4–10 pH range. AC- and DC-based current measurements were performed to study the sensing mechanism. Conductance-based pH measurement, a unique and precise technique, yielded a high sensitivity of 4.59 mS/pH. The proposed nanosensors, which are low cost and easy to fabricate and use, exhibited rapid response, long-term stability, high sensitivity (in both potentiometric as well as conductance-based measurements), and high repeatability.

Self-Enhanced Carbonized Polymer Dots for Selective Visualization of Lysosomes and Real-Time Apoptosis Monitoring.

SummaryProtons are highly related to cell viability during physiological and pathological processes. Developing new probes to monitor the pH variation could be extremely helpful to understand the viability of cells and the cell death study. Carbonized polymer dots (CPDs) are superior biocompatible and have been widely applied in bioimaging field. Herein, a new type of extreme-pH suitable CPDs was prepared from citric acid and o-phenylenediamine (CA/oPD-CPDs). Due to the co-existence of hydrophilic and hydrophobic groups, CA/oPD-CPDs tend to aggregate in neutral condition with a dramatic decrease of fluorescence, but disperse well in both acidic and alkaline conditions with brighter emission. This specialty enables them to selectively illuminate lysosomes in cells. Moreover, CA/oPD-CPDs in the cytoplasm could serve as a sustained probe to record intracellular pH variation during apoptosis. Furthermore, CA/oPD-CPDs present a continuous fluorescence increase upon 2-h laser irradiation in living cells, underscoring this imaging system for long-term biological recording.

Mechanisms behind excitation- and concentration-dependent multicolor photoluminescence in graphene quantum dots.

Despite numerous efforts, the mechanism behind multicolor photoluminescence (PL) in graphene quantum dots (GQDs) is still controversial. A deep insight into the origin of the multicolor emissions in GQDs is quite necessary for modulating their luminescence to facilitate the better use of this fluorescent material. Herein, GQDs with amino, carboxyl, and ammonium carboxylate groups were synthesized. The as-prepared GQDs exhibited intriguing excitation- and concentration-dependent multicolor PL characteristics. By regulating the excitation wavelength or concentration of GQDs, specific luminescence colors including blue, cyan, green, yellow, and even orange can be obtained. Systematic structural and optical studies indicated that the graphene basal plane and different functional groups dominantly exhibited nN 2P-σ*, π-π*, nO 2p-π* (-COOH), nO 2p-π* (-COO-) and nN 2p-π* electronic transitions, which appeared as multi-fluorescent centers that gave rise to the excitation-dependent multicolor PL. The occurrence of different types of electronic transitions and their color emissions were proved by pH-dependent PL measurements. In addition, systematic optical and morphology analyses revealed that GQDs could self-assemble into J-type aggregates with different morphologies and sizes as the concentration increased, and the observed concentration-dependent multicolor PL can be ascribed to aggregation-mediated energy level reconstruction in GQDs. Our findings further suggest that the competition among various fluorescent centers and self-aggregation processes dominated the luminescent properties of GQDs. This work will contribute to understand the origins of excitation- and concentration-dependent multicolor emissions in GQDs, which is also highly instructive for broadening the application fields of GQDs.

Stimuli-responsive photoluminescence soft hybrid microgel particles: synthesis and characterizations

Stimuli-responsive (pH and temperature sensitive) photoluminescence hybrid particles are prepared using oppositely charged cationic microgels of Poly(N-isopropyl polyacrylamide) (PNIPAm) and anionic cadmium sulfide (CdS) quantum dots (QDs). A facile synthetic strategy such as in situ/post incorporation of QDs along with pH tuneable electrostatic interactions is optimized to obtain hybrid microgels with maximum photoluminescence. Transmission electron microscope (TEM), fluorescence spectroscopy and dynamic light scattering (DLS) methods are used for characterizing the synthesized hybrid particles. TEM studies directly confirm the successful loading of QDs onto microgels whereas fluorescence spectroscopy reveals higher photo luminosity of the hybrid microgels prepared via in situ compared to post incorporation method. The pH-dependent photoluminescence supported by temperature-dependent swelling studies using DLS suggest that the hybrid microgels prepared at low pH have greater fluorescence with less thermoresponsivity and these behaviors follow an opposite trend with increasing pH. Further, these results are compared with the hybrid microgels prepared using similar charged anionic microgels and anionic quantum dots under same experimental condition (via in situ) and suggest that maximum photoluminescence can be achieved only through oppositely charged species.

Quantitative Measurements of the pH-Sensitive Quantum Yield of Fluorophores in Mesoporous Silica Thin Films Using a Drexhage-Type Experiment

The photoluminescence quantum yield characterizes the performance of emitters for applications in optical devices, as reporters or probes in material and analytical sciences, and for sensing applications. Quantum yield measurements are challenging for luminescent molecules and nanocrystals immobilized in thin films for many sensor applications, particularly if spatially resolved quantitative luminescence information is desired. We show here that a Drexhage-type experiment, where a silver-coated millimeter-sized sphere is used to modify the local density of states, can provide an elegant approach to counter this challenge. As a representative example of the potential of this method, we measure the pH-dependent photoluminescence quantum yield of fluorescein isothiocyanate bound to a thin mesoporous silica film. The results were compared with those of the studies on the pH dependence of the same dye in solution. We found that our approach can link single fluorophore studies to ensemble measurements and pave ...

Three coordination polymers constructed from a multidentate N-donor ligand, polycarboxylate anions and Zn(II)/Ag(I)/Mn(II) ions: synthesis, structures, characterization and pH-dependent photoluminescence

Three new coordination polymers, namely {[Zn(5-OH-bdc)(2,4′-tmbpt)]·5H2O}n (1), [Ag3(L)(2,4′-tmbpt)2]n (2), and [Mn3(L)2(2,4′-tmbpt)2]n (3), have been prepared under hydrothermal condition (2,4′-tmbpt = 1-((1H-1,2,4-triazol-1-yl)methyl)-3-(4-pyridyl)-5-(2-pyridyl)-1,2,4-triazole, 5-OH-H2bdc = 5-hydroxy-1,3-benzenedicarboxylic acid, and L = 5-(4-carboxybenzyloxy)isophthalic acid). The structures of compounds 1–3 have been determined by single-crystal X-ray diffraction analysis. The structures of 1 and 3 have been characterized by elemental analysis, infrared spectra, powder X-ray diffraction analysis and thermogravimetric analysis. Compound 1 displays a 2D 4-connected sql net with the Schlafli symbol of (44·62), which is further connected by the intermolecular hydrogen bonds to generate a 3D supramolecular architecture. Compound 2 is a 3D two fold interpenetrating framework with the Schlafli symbol of (3·6·7)(32·83·9)(3·64·72·8·9·10). Compound 3 is a 3D (3,6,6)-connected framework with the Schlafli symbol of (3·62)(34·66·85)(33·68·83·10). The pH-dependent photoluminescent spectra of compound 1 have been investigated, and the pH-dependent photoluminescent mechanism has been discussed and proved by UV–Vis absorption spectra. Three compounds based on a multidentate N-donor ligand, Zn(II)/Ag(I)/Mn(II) ions and polycarboxylic acids have been synthesized under the hydrothermal conditions. The structures of these compounds have been characterized comprehensively. The pH-dependent photoluminescent spectra of compound 1 have been investigated. Moreover, the pH-dependent photoluminescent mechanism has been discussed and proved by UV–Vis absorption spectra.

Facile synthesis and photoluminescence mechanism of green emitting xylose-derived carbon dots for anti-counterfeit printing

The realization of long-wavelength emissions and thoroughly understanding of photoluminescence (PL) mechanism are still huge challenges to be addressed for further development of carbohydrate-derived carbon dots (CDs). Herein, novel CDs with excellent green emission were synthesized from a precursor comprising of xylose and m-phenylenediamine in phosphoric acid solution via a simple microwave-assisted hydrothermal method. The PL efficiency was dependent on the combination of different acids and the precursor as well as the synthesis temperature, while the PL properties were mainly related to the acid base environment of precursor. Optical performance and structural characterization revealed that the oxygen-containing groups and protonated amino groups were mainly responsible for the blue and green emission properties, respectively. Moreover, a multi-center fluorescent mechanism on the different PL performances of xylose-derived CDs was proposed and verified by further investigating the pH-dependent PL behaviors. Based on the switching between green and blue emission in different pH regions, the xylose-derived CDs solution was demonstrated to be a promising fluorescent ink for anti-counterfeit printing. This research open a door for value-added utilization of xylose, as well as providing a facile and green strategy for large-scale preparation of CDs.

PH-Dependent photoluminescence “switch-on” nanosensors composed of silver nanoparticles and nitrogen and sulphur co-doped carbon dots for discriminative detection of biothiols

Very close structure and property similarities among biothiols, such as glutathione (GSH), cysteine (Cys), and homocysteine (Hcy), present a major challenge to achieve their discriminative detection. In this contribution, a nanomaterial surface energy transfer (NSET) system was established to discriminate GSH from Cys and Hcy with the photoluminescence (PL) "switch-on" response. The nanosensor was constructed using nitrogen and sulphur co-doped carbon dots (N,S-CDs) and silver nanoparticles (AgNPs) through assembling an energy transfer-based quenching system, featuring the pH-promoted distinct PL "switch-on" response. Under neutral conditions, only Cys and Hcy can encapsulate AgNPs, leaving little chance for N,S-CD binding on the surface of AgNPs, which can lead to PL signal recovery, and the total quantity of Cys and Hcy can be determined. However, at pH 3.0 all three kinds of biothiols can lead to the PL signal recovery and the total quantity of GSH, Cys, and Hcy can be determined, due to the similar slopes and intercepts of their calibration curves. Thus, the concentration of GSH could be further calculated and the strategy was successfully applied for the detection of GSH in human serum, demonstrating its potential in bioanalysis and significance in addressing biological and medicinal requirements.

Synthesis of Longer‐Wavelength‐Emissive Carbon Quantum Dots for WLEDs and Investigation of Their Photoluminescence Properties

AbstractThe synthesis of highly fluorescent longer wavelength emissive carbon quantum dots (CQDs) is still a great challenge as well as an important issue, hindering its application in white light emitting diode (WLED). Herein, we have demonstrated a simple one‐pot solvothermal route for the synthesis of nitrogen and sulphur co‐doped CQDs using pyromellitic acid (PA), diethylenetriamine (DETA) and thiourea. At the specific molar ratio of the precursors, the orange color emissive CQDs (O‐CQDs) are synthesized and possess longer wavelength emission (611 nm). Additionally, O‐CQDs exhibits temperature and pH dependent photoluminescence (PL) properties. The O‐CQDs shows color correlated temperature (CCT) of 1745 K with high color purity of 98%. Simultaneously, as‐synthesized nitrogen and sulphur co‐doped CQDs shows concentration‐dependent PL properties, which display not only the brightness with stable orange color but also possess range of multicolour emission at different PL excitation. Solid host matrix of O‐CQDs is fabricated with poly‐(methyl methacrylate) (PMMA) chloroform solution and orange emissive property is investigated by wrapping the film over LED (InGaN) bulb. These studies will open up new opportunities to explore the tunable optical properties of carbon quantum dots and its applications in WLED devices.

On the intriguing emission characteristics of size tunable carbon dots derived from functionalized multi-walled carbon nanotubes

Abstract Aqueous dispersions of carbon dots (C-dots) showing size tunable multi-colours in day light are obtained through surface functionalization of multi-walled carbon nanotubes (MWCNTs). The size of the C-dots varies with the functionalization temperature and smaller C-dots are found to be formed, at higher temperatures. The average size of the C-dots synthesized at temperatures of 100 °C, 80 °C and 60 °C lies in the range 6 nm–18 nm, based on transmission electron microscopy (TEM) images. The C-dots synthesized at these temperatures exhibit blue, green and golden yellow colours in day light without being excited by any external source. The structural characterizations using X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy confirm the presence of a high density of functional groups on the surfaces of the C-dots, giving rise to high aqueous solubility. Compared to the Raman spectrum of the MWCNTs, the spectrum of the C-dots shows a significant increase in the intensity of the defect related D band with respect to that of the crystalline G-band, which indicates the presence of a high concentration of defects and the formation of smaller sp2 carbon clusters. The synthesized C-dots exhibit size dependent optical absorption and emission characteristics. The intriguing optical properties of these C-dots, which include the multi-colour emission at day light and the excitation wavelength and the pH dependent photoluminescence emission, can be mainly attributed to the formation of sp2 clusters of different sizes embedded in the sp3 carbon matrix.

PH controlled green luminescent carbon dots derived from benzoxazine monomers for the fluorescence turn-on and turn-off detection

Emerging carbon dots (CDs) are widely used as fluorescent probes in biological and environmental fields, nevertheless, the control of CDs based on different detection mechanisms is rarely reported. In this paper, green luminescent CDs (G-CDs) were prepared by a facile hydrothermal treatment of benzoxazine monomers (BZM). The obtained G-CDs showed pH dependent photoluminescence, which could be designed as fluorescence turn-on and turn-off sensors. The G-CDs exhibited weak photoluminescence at pH = 7.0 and could be turned on by Zn(II) selectively with the limitation of 0.32 μM in the concentration range from 1 to 100 μM. When pH = 10.0, Cr(VI) could quench the strong fluorescence of G-CDs efficiently, and the limit of detection was 0.99 μM with a linear range of 1–50 μM. Furthermore, the fluorescence turn-on and turn-off performance of G-CDs was attributed to the intramolecular charge transfer (ICT) of Zn(II) and the inner filter effect (IFE) of Cr(VI), respectively. The excellent probes were successfully applied for the detection of Zn(II) in biological system and Cr(VI) in environment.

A nanocomposite probe consisting of carbon quantum dots and phosphotungstic acid for fluorometric determination of chromate(VI) with improved selectivity.

A fluorometric quenching assay is described for the determination of chromate(VI) by using a nanocomposite probe consisting of carbon quantum dots (CQDs) and phosphotungstic acid (HPW). The stable nanoprobe was synthesized via hydrothermal carbonization of glucose in the presence of HPW. HPW promotes the dehydration and carbonization and acts as an "electronic receptor". It blocks the radiative electron/hole recombination in the CQDs and leads to a product whose fluorescence (with excitation/emission peaks at 360/463nm) is quenched. The CQD/HPW was characterized by transmission electron microscopy, FT-IR spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, UV-vis absorption and fluorescence spectroscopy to characterize their surface morphology, functional groups and elemental composition, crystal structure and optical properties. The nanocomposite is nearly mono-disperse with an average particle diameter of 1.7nm, and displays excitation wavelength-dependent and pH-dependent photoluminescence. Fluorescence drops on addition of chromate(VI) due to an inner filter effect. The ability of receiving electron for HPW can hinder the electron transfer from CQD/HPW to other metal ions, so the nanocomposite showed excellent selectivity towards chromate(VI). Fluorescence drops linearly with the concentration of chromate(VI) in the range from 2 to 80μM, with a limit of detection of 0.16μM. Graphical abstract Hydrothermal carbonization preparation of carbon quantum dots and phosphotungstic acid nanocomposite probe for fluorometric determination of chromate(VI) based on inner filter effect.

The effects of QD stabilizer structures on pH dependence, fluorescence characteristics, and QD sizes

CdTe quantum dots (QDs) were prepared in aqueous solution using various mercaptocarboxylic acids such as mercaptopropionic acid (MPA), mercaptoethylamine (MEA), and L-cysteine (L-Cys) as stabilizing agents. The pH control of QD suspensions is essential when using fluorescent QDs in biological systems. The pH-dependent photoluminescence (PL) of MPA- and L-Cys-stabilized CdTe QDs was systematically investigated. Experimental results showed that CdTe-L-Cys is effective within a wider pH range than CdTe-MPA and CdTe-MEA, although it is ineffective in the vicinity of the isoelectric point, which makes CdTe-L-Cys quantum dots potentially suitable as a biological labeling probe under physiological conditions. Meanwhile, the effect of stabilizer structure on QD size was studied by comparing CdTe-MPA and CdTe-L-Cys QDs using x-ray diffraction, x-ray photoelectron spectroscopy and transmission electron microscopy. The results indicated that the coordination of amines with cadmium reduced the activity of Cd precursors, and the coordination of S with the CdTe QDs affected their crystal structure, resulting in the formation of relatively large CdTe-L-Cys quantum dots.

Investigation on the pH-independent photoluminescence emission from carbon dots impregnated on polymer matrix.

Highly luminescent, polymer nanocomposite films based on poly(vinyl alcohol) (PVA), and monodispersed carbon dots (C-dots) derived from multiwalled carbon nanotubes (MWCNTs), as coatings on substrates as well as free standing ones are obtained via solution-based techniques. The synthesized films exhibit pH-independent photoluminescence (PL) emission, which is an advantageous property compared with the pH-dependent photoluminescence intensity variations, generally observed for the C-dots dispersed in aqueous solution. The synthesized C-dots and the nanocomposite films are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), ultraviolet (UV)-visible spectroscopy and photoluminescence spectroscopy (PL) techniques. The TEM image provides clear evidence for the formation of C-dots of almost uniform shape and average size of about 8nm, homogeneously dispersed in aqueous medium. The strong anchoring of C-dots within the polymer matrix can be confirmed from the XRD results. The FTIR spectral studies conclusively establish the presence of oxygen functional groups on the surfaces of the C-dots. The photoluminescence (PL) emission spectra of the nanocomposite films are broad, covering most part of the visible region. The PL spectra do not show any luminescence intensity variations, when the pH of the medium is changed from 1 to 11. The pH-independent luminescence, shown by these films offers ample scope for using them as coatings for designing diagnostic and imaging tools in bio medical applications. The non-toxic nature of these nanocomposite films has been established on the basis of cytotoxicity studies.