pH-sensitive Photoluminescence Research Articles

Mechanism study of Dual-Emission ratiometric fluorescent pH-Sensitive carbon quantum dots and its application on mornitoring enzymatic catalysis

Carbon dots (CQD) have received significant attention as a novel ratiometric fluorescent pH nanoprobe, owing to their favorable optical properties and excellent biocompatibility. Despite their appealing features, the precise mechanism behind the pH-sensitive photoluminescence of CQDs remains to be fully understood. This study endeavors to unravel the mechanism underlying the pH-responsive ratiometric fluorescence in dual-emission CQDs, synthesized through a one-step hydrothermal method using o-phenylenediamine and oxalic acid as precursors. The resultant CQDs exhibit inherent dual-emission at wavelengths of 383 nm and 566 nm, with the ratiometric fluorescence response tailored by the ratio of precursors, providing a robust tool for pH sensing across a range of 2 to 6. Detailed characterizations, including chemical, morphological, and optical analyses, alongside theoretical insights from time-dependent density functional theory (TD-DFT), elucidate the mechanism underlying the pH-dependent luminescence, attributed to the electron cloud transmission between amide and adjacent carboxyl groups on the CQD surface. The superior performance of these CQDs in real-time pH monitoring is demonstrated through their application in glucose oxidase-catalyzed reactions, showcasing their potential as efficient, reliable nanoprobes for biomedical research and diagnostic applications.

Bovine serum albumin protected Cd8 nanoclusters as efficient two-photon absorbers for near-infrared excited fluorescence imaging of intracellular pH

Metal nanocluster-based fluorescent sensors have made great progress in the field of analytical chemistry, but their practical applications in biological sensing were limited by poor stability, short wavelength excitation and low quantum yield. It is of great significance to develop NIR-excited metal nanocluster-based fluorescence sensors with high quantum yield. In contrast to noble metal nanoclusters (e.g., Au, Ag, and Pt), stable and affordable fluorescent metal nanoclusters have received relatively less research attention, with the exception for Cu nanoclusters. Herein, we developed a rapid one-pot route to synthesize cadmium nanoclusters employing bovine serum albumin (BSA) as both reducing and templating agent. The obtained Cd8NCs exhibited bright fluorescence emission with quantum yield of 13.8%. Of particular interest, the Cd8NCs showed remarkable capability as two-photon absorbers, with a two-photon absorption (TPA) cross section up to 1588 GM (excited at 730 nm). The TPA cross section was higher than that of BSA-protected AuNCs, and even exceeded that of conventional dyes by two orders of magnitude. In addition, the Cd8NCs exhibited pH-sensitive photoluminescence, and the pH-responsive mechanism was revealed by time-resolved fluorescence spectroscopy, transient absorption spectroscopy, etc. Benefiting from protein coating, Cd8NCs showed excellent biocompatibility and were successfully utilized as two-photon excited fluorescence sensors for intracellular pH.

PH-Sensitive Fluorescence Emission of Boron/Nitrogen Co-Doped Carbon Quantum Dots

Carbon quantum dots (CQDs) with their strong photoluminescence (PL) activity, high biocompatibility, robust stability, low cytotoxicity, and flexible surface structures have been employed in many fields including chemical sensing, biosensing, photocatalyst, energy storage, and biomedical applications. Of note, CQDs present an intrinsic pH-sensitive PL nature indicating their intense potential for pH-mediated sensing and imaging. Despite the numerous studies performed in the last two decades, the pH-sensitive PL mechanism of CQDs is still under debate and must be clarified to overcome the limitations in practical applications. Therefore, in this report, we performed a systematical study to determine the pH-sensitive PL nature of boron/nitrogen co-doped CQDs (B/N CQDs). In the first part, B/N CQDs with a strong blue emission were fabricated via a hydrothermal synthesis procedure. B/N-CQDs showed a strong blue PL emission with high quantum yield and excitation-dependent nature. Under the low pH conditions (pH 3), B/N-CQDs exhibited a robust green fluorescence emission with a significant red-shift (48 nm) and the loss of the excitation-dependent nature. The change in PL nature originated from the protonation of surface groups, a decrease in negative surface charge (from −20.6 to −1.23 eV), and finally, aggregation of the nanostructure (the size of CQDs from 4.8 to 7.5 nm). However, in the case of alkaline conditions, the deprotonation surface groups significantly enhanced the surface charge and led to the emergence of a negative ‘protective shell’ with a zeta potential of −71.3 eV. In a high pH medium (pH 13), PL spectra showed the loss of excitation-dependent features and a red-shift (35 nm) in emission peak maxima with lower intensity. This report provides significant progress in the clarification of the pH-sensitive PL mechanism of CQDs. We envision that the proposed CQDs would provide unique opportunities in the fabrication of novel pH sensor systems and fluorescence imaging where a wide range of pH sensitivity is required.

Origins of the pH-Responsive Photoluminescence of Peptide-Functionalized Au Nanoclusters.

Ultrasmall peptide-protected gold nanoclusters are a promising class of bioresponsive material exhibiting pH-sensitive photoluminescence. We present a theoretical insight into the effect peptide-ligand environment has on pH-responsive fluorescence, with the aim of enhancing the rational design of gold nanoclusters for bioapplications. Employing a hybrid quantum/classical computational methodology, we systematically calculate deprotonation free energies of N-terminal cysteine amine groups in proximity to the inherently fluorescent core of Au25(Peptide)18 nanoclusters. We find that subtle changes in hexapeptide sequence alter the electrostatic environment and significantly shift the conventional N-terminal amine pKa expected for amino acids free-in-solution. Our findings provide an insight into how the deprotonation equilibrium of N-terminal amine and side chain carboxyl groups cooperatively respond to solution pH changes, explaining the experimentally observed, yet elusive, pH-responsive fluorescence of peptide-functionalized Au25 clusters.

Strict Twice Iterative Optimization Strategy to Synthesize Ultrabright Fluorescent Carbon Dots for UV and pH Dual-Encryption Fluorescent Ink.

In this work, ultrabright fluorescent carbon dots (U-CDs) were synthesized by using a strict twice iterative optimization strategy. Their relative photoluminescence (PL) quantum yield is close to 100%, exceeding most of the reported fluorescent CDs and greatly boosting the practical applications of fluorescent CDs in many fields. Then serving as fluorescent anti-counterfeiting ink was taken as an example to briefly introduce the application of the U-CDs. The PL emission of the U-CDs is quenched at the range of pH < 4 or pH > 11 and restored at the range of pH = 5–10. This pH-sensitive PL feature allows the U-CDs to be used as fluorescent ink for pH and UV dual information encryption. The written or printed information is invisible under daylight but visible under UV light. After acid treatment or alkali treatment, the information is invisible even under a UV lamp but visible after neutralization treatment. This work provides a standardized scheme for optimizing the synthesis conditions of fluorescent CDs and paves the way for large-scale production of high-performance fluorescent CDs.

Quantitatively Switchable pH-Sensitive Photoluminescence of Carbon Nanodots.

pH sensing plays a key role in the life sciences as well as the environmental, industrial, and agricultural fields. Carbon nanodots (C-dots) with small size, low toxicity, and excellent stability hold great potential in pH sensing as nanoprobes due to their intrinsic pH-sensitive photoluminescence (PL). Nonetheless, the undesirable sensitivity and response range of C-dot PL toward pH cannot meet the requirements of practical applications, and the unclear pH-sensitive PL mechanism makes it difficult to control their pH sensitivity. Herein, the quantitative correlation of pH-sensitive PL with specific surface structures of C-dots is uncovered for the first time, to our best knowledge. The association of carboxylate and H+ increases the ratio of nonradiation to radiation decay of C-dots through excited-state proton transfer, resulting in the decrease of PL intensity. Meanwhile, the dissociation of α-H in β-dicarbonyl forming enolate increases the extent of delocalization of the C-dots conjugated system, which induces the PL broadening to the red region and a decreasing intensity. Based on the understanding of the pH-sensitive PL mechanism, the pH-sensitive PL of C-dots can be switched by quantitative modulation of carboxyl and β-dicarbonyl groups to achieve a desirable pH response range with high sensitivity. This work contributes to a better understanding of the pH-sensitive PL of C-dots and therefore presents an effective strategy for controllably tuning their pH sensitivity, facilitating the rational design of C-dot-based pH sensors.

A Mini Review on pH-Sensitive Photoluminescence in Carbon Nanodots.

Carbon nanodots (C-dots) with sp2/sp3 framework and diameter of <10 nm contain abundant functional groups or polymers on their surface. C-dots have attracted immense attention because of their unique optical properties, excellent biocompatibility, facile preparation, and low cost. With these merits, C-dots have been used in a wide range of applications including sensing, bioimaging, catalysis, and light-emitting devices. C-dots exhibit good optical properties, such as tunable emission wavelength, good photostability, nonblinking, up-conversion emission, etc. Of note, C-dots show intrinsic pH-sensitive photoluminescence (PL), indicating their great potential for pH sensing, especially in biotic pH sensing. In this review, we systematically summarize the pH-sensitive PL properties and the pH-sensitive PL mechanism, as well as recent research progress of C-dots in pH sensing. The current challenges of pH-sensitive C-dots and their future research focus are also proposed here. We anticipate this review might be of great significance for understanding the characteristics of pH-sensitive C-dots and the development of photoluminescent nanomaterials with pH-sensitive properties.

Facile synthesis of amine-functionalized graphene quantum dots with highly pH-sensitive photoluminescence

ABSTRACTGraphene quantum dots (GQDs) as promising materials have gained increasing attention due to their optical and chemical properties, but the complicated synthetic procedures restrict their large-scale application. Here we report a novel one-pot hydrothermal method to synthesize the GQDs with bifunctional groups (-OH and -NH2) using a polycyclic aromatic hydrocarbon as the precursor, in which 1,5-dinitronaphthalene was graphitized in aqueous ammonium solution. The as-synthesized GQDs with single-layer structures have a size distribution of 1–3 nm and an average value of 1.5 nm. The alkaline species play a crucial role in the formation of amine-functionalized GQDs. More importantly, the as-synthesized GQDs exhibit a distinct pH-sensitive photoluminescence and the fluorescence colors vary from bluish green to yellow with pH value from 5 to 10, which can be attributed to the protonation and deprotonation of the amine-groups in acid or alkaline solution.

One-pot green synthesis of oxygen-rich nitrogen-doped graphene quantum dots and their potential application in pH-sensitive photoluminescence and detection of mercury(II) ions

Nitrogen doping has been a powerful method to modulate the properties of carbon materials for various applications, and N-doped graphene quantum dots (GQDs) have gained remarkable interest because of their unique chemical, electronic, and optical properties. Herein, we introduce a facile one-pot solid-phase synthesis strategy for N-doped GQDs using citric acid (CA) as the carbon source and 3,4-dihydroxy-l-phenylalanine (l-DOPA) as the N source. The as-prepared N-GQDs with oxygen-rich functional groups are uniform with an average diameter of 12.5nm. Because of the introduction of nitrogen atoms, N-GQDs exhibit excitation-wavelength-independent fluorescence with the maximum emission at 445nm, and a high quantum yield of 18% is achieved at an excitation wavelength of 346nm. Furthermore, a highly efficient fluorosensor based on the as-prepared N-GQDs was developed for the detection of Hg2+ because of the effective quenching effect of metal ions via nonradiative electron transfer. This fluorosensor exhibits high sensitivity toward Hg2+ with a detection limit of 8.6nM. The selectivity experiments reveal that the fluorescent sensor is specific for Hg2+. Most importantly, the practical use of the sensor based on N-GQDs for Hg2+ detection was successfully demonstrated in river-water samples.

Waste frying oil as a precursor for one-step synthesis of sulfur-doped carbon dots with pH-sensitive photoluminescence

Proper disposal of waste frying oil (WFO) is an important waste-management concern. In this paper, we develop a facile method to reuse WFO for one-step synthesis of sulfur-doped carbon dots (S-C-dots) with the assistance of concentrated sulfuric acid. The as-synthesized S-C-dots are uniform in size and show partial disordered graphite-like structure. Different from the doping-free or nitrogen-doped carbon dots, the S-C-dots perform a strong ultraviolet emission at 378nm due to successful sulfur-doping. Noticeably, the S-C-dots exhibit a distinct pH-sensitive feature and the intensity of photoluminescence increases linearly in the pH range from 3 to 9. Furthermore, possessing fascinating optical properties, high photostability, and low cytotoxicity, the S-C-dots have served as fluorescent probes for cell imaging.

Nitrogen-doped carbon dots derived from polyvinyl pyrrolidone and their multicolor cell imaging

Nitrogen-doped carbon dots (N-CDs) with a high quantum yield of 19.6% were prepared by calcining polyvinyl pyrrolidone (PVP, K-30), and then modified with 4,7,10-trioxa-1,13-tridecanediamine. The as-prepared N-CDs exhibited excitation-dependent and pH-sensitive photoluminescence. Transmission electron microscopy and Raman spectra demonstrated the graphitic structure of the N-CDs. Fourier transform infrared spectroscopy and x-ray diffraction studies revealed successful passivation and the presence of hydrophilic groups on the surface. Importantly, such modified quantum dots acted as good multicolor cell imaging probes due to their excellent fluorescent properties, low cytotoxicity and fine dispersity.

Electrochemical synthesis of photoluminescent carbon nanodots from glycine for highly sensitive detection of hemoglobin

An electrochemical approach has been employed for the preparation of photoluminescent carbon nanodots (C-dots) from glycine under alkaline conditions. The formation of C-dots is through electro-oxidation, electro-polymerization, carbonization, and passivation. The as-prepared C-dots possess excitation-wavelength-dependence and pH sensitive photoluminescence (PL) properties and are stable in solution containing high salt concentration (up to 500 mM NaCl). Detection of hemoglobin using C-dots has been demonstrated through a fluorescence resonance energy transfer (FRET) process. The PL intensity (excitation/emission wavelengths 365/440 nm) of C-dots is inversely proportional to the concentration of hemoglobin over a range of 0.05–250 nM (r = 0.99), with a limit of detection (signal-to-noise ratio 3) of 30 pM. We have validated this assay by determining the concentrations of hemoglobin in five representative diluted blood samples, with results being in good agreement with that obtained by using a commercial hemoglobin meter. The water-dispersible and photostable C-dots have been applied to obtain bloodstained images and fingerprints.

PH-sensitive photoluminescence for aqueous thiol-capped CdTe nanocrystals

pH-sensitive photoluminescence (PL) is an important property of aqueous nanocrystals(NCs) towards NCs-based intelligent applications. Previous works mainly focusedon the effect of pH during NC growth process on PL of the aqueous NCs. Theeffect of pH during application process on PL of as-prepared NCs is still not fullyunderstood. In this work, we brought out a general mechanism for the pH-sensitive PLbehaviors of as-prepared aqueous CdTe NCs capped by aqueous thiol ligands, such ascarboxylic-acid-terminated 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA)ligands, hydroxyl-terminated 1-thioglycerol (TG) ligands and amine-terminated2-mercaptoethylamine (MA) ligands. A major contribution of this work is finding the keyrole of ligand terminal groups in the diffuse process of free Cd–ligand complexestoward NCs. This terminal group effect is the main reason for PL alteration ofNCs during pH adjustment process. Besides the terminal group effect, PL ofaqueous NCs is also affected by the aggregation effect, the thiol group effect andthe counter ion effect. These effects make different contributions to PL of NCsat different pH ranges. By using this mechanism, we successfully explained thecomplex pH-sensitive PL behaviors of MPA, TGA, TG and MA-capped CdTe NCs.

Fluorescent carbon nanoparticles: electrochemical synthesis and their pH sensitive photoluminescence properties

Highly fluorescent carbon nanoparticles were synthesized directly from ethanol by a one-step sodium hydroxide-assisted electrochemical treatment. These nanoparticles could emit bright and colorful photoluminescence covering the entire visible spectral range and had excellent up-conversion fluorescent, stable ionic, long fluorescence lifetime and pH-sensitive photoluminescence properties.

Water-soluble polyaniline with strong photoluminescence in physiological pH environment

Polyaniline (PANI) can show strong photoluminescence (PL) and well dispersed ability in water after simple refluxing with H 2SO 4 (sulfonated polyaniline, SPANI). This kind of fluorescent SPANI exhibits pH sensitive PL properties, especially, the strongest PL intensity is obtained in physiological pH environment (pH 7–8). The pH sensitive PL properties of SPANI are mainly attributed to the structure transition under different pH conditions.

Temperature and pH sensitive photoluminescence of riboflavin-methyl cellulose hydrogel: towards AND molecular logic gate behaviour

Download options Please wait... Article information DOI https://doi.org/10.1039/B909008H Article type Paper Submitted 06 May 2009 Accepted 10 Jul 2009 First published 05 Aug 2009 Download Citation Soft Matter, 2009,5, 3992-3996 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Temperature and pH sensitive photoluminescence of riboflavin-methyl cellulose hydrogel: towards AND molecular logic gate behaviour A. Saha, S. Manna and Arun. K. Nandi, Soft Matter, 2009, 5, 3992 DOI: 10.1039/B909008H To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Abhijit Saha Swarup Manna Arun. K. Nandi