Surface modifications of carbon nanodots reveal the chemical source of their bright fluorescence

Assessing the efficacy of carbon nanodots derived from curcumin on infectious diseases

INTRODUCTION

The threat of new, emerging, and multidrug-resistant microbes is increasing which has created the necessity for new antimicrobials. In this regard, nanotechnology can be an alternative for the treatment of infectious microbes. Curcumin has been used since ancient times as antimicrobials; however, it has limitations due to its less aqueous solubility, bioavailability, and biocompatibility. This problem can be solved by curcumin-derived carbon nanodots, which are emerging antimicrobials of <10 nm size, water-soluble, biocompatible, less toxic, and fluorescent.

AREAS COVERED

The review discusses the application of curcumin-derived carbon nanodots against various pathogenic microbes including bacteria and dreaded viruses like SARS-CoV-2. In addition, the role of curcumin carbon nanodots in biolabelling of pathogenic microbes, mechanism of action, bioimaging, and therapy has been critically examined.

EXPERT OPINION

Carbon nanodots play an important role in combating pathogenic microbes by early diagnosis, bioimaging, nanocarrier for antimicrobial drugs, and therapy of infectious diseases. Curcumin carbon nanodots have already demonstrated their benefits of being water soluble, bioavailable, and biocompatible. However, more thorough research is needed to understand the efficacy and safety of curcumin carbon nanodots. In the future, curcumin-derived carbon nanodots can be used as alternative antimicrobial agents to fight microbial infections including multidrug-resistant microbes.

A switchable green emitting dye and its phenomenal properties: implications for the photoluminescence features of carbon dots

New molecular fluorophores are constantly being discovered in post-synthetic mixtures of carbon dots (CDs), prompting researchers to elucidate their role in the optical properties of these nanomaterials. It has been reported that the green-emitting fluorophore that forms during the synthesis of popular citric acid/urea CDs is HPPT (4-hydroxy-1H-pyrrolo[3,4-c]pyridine-1,3,6(2H,5H)-trione). However, due to the low concentration of HPPT-like molecules within the structure of CDs, their actual binding and contribution to the optical properties of CDs has not so far been convincingly confirmed. In this joint experimental and quantum chemical study, we show that HPPT is a strong acid and only its anionic form, HPPT-, present in solutions of pH 0-10, is emissive. Next, its fluorescence can be switched off rapidly in strongly alkaline environments as a result of HPPT- hydrolysis, leading to the opening of its pyrrole ring and formation of CDPC (3-carbamoyl-2,6-dihydroxypyridine-4-carboxylic acid), existing as the CDPC2- dianion under these conditions. Eventually, we found that the ring opening hydrolysis is reversible and the green emission may be restored in acidic environments. The kinetics and mechanism of this hydrolysis were also revealed. The optical features of citric acid (CA)-urea CDs under various conditions were compared with a simpler CD system prepared by treating the CDs obtained from CA solely with HPPT- (HPPT@CDs). Our results indicate the feasibility of the post-synthetic modification of HPPT- present in the structures of CA-urea CDs and HPPT@CDs. Without HPPT- they emit blue fluorescence only. Thus, this makes the nanosystem switch the PL emission colour reversibly from green to blue owing to the opening and closing of the pyrrole ring in HPPT-like molecules. More importantly, the latter process may be considered a first step toward genuine fine tuning of the PL emission colour from CDs. These findings are of general importance to the further development of citric acid-based CDs with tailored properties.

Green microwave-prepared N and S Co-doped carbon dots as a new fluorescent nano-probe for tilmicosin detection

A straightforward, rapid, and selective fluorescent probe for determination of tilmicosin has been developed based on novel nitrogen and sulfur co-doped CDs (NS-CD). The NS-CDs were synthesized, for the first time, through green, simple one step microwave pyrolysis in only 90 s using glucose as carbon source and l-cysteine as nitrogen and sulfur source. This proposed synthesis method was energy-efficient and resulted in NS-CDs with high production yield (54.27 wt%) and narrow particle size distribution. Greenness of NS-CDs synthesis method was assessed using EcoScale and was proven to be excellent green synthesis. The produced NS-CDs were applied as a nano-probe for determination of tilmicosin in its marketed formulation and milk based on dynamic quenching mechanism. The developed probe showed a good performance for tilmicosin detection in marketed oral solution and pasteurized milk and linearity range of 9-180 μM and 9-120 μM, respectively.

Multifunctional role of carbon dot-based polymer nanocomposites in biomedical applications: a review

Carbon-based 0D materials have shown tremendous potential in the development of biomedical applications of the next generation. The astounding results are primarily motivated by their distinctive nanoarchitecture and unique properties. Integrating these properties of 0D carbon nanomaterials into various polymer systems has orchestrated exceptional potential for their use in the development of sustainable and cutting-edge biomedical applications such as biosensors, bioimaging, biomimetic implants and many more. Specifically, carbon dots (CDs) have gained much attention in the development of biomedical devices due to their optoelectronic properties and scope of band manipulation upon surface revamping. The role of CDs in reinforcing various polymeric systems has been reviewed along with discussing unifying concepts of their mechanistic aspects. The study also discussed CDs optical properties via the quantum confinement effect and band gap transition which is further useful in various biomedical application studies.

Enhancing the photoluminescence and cellular uptake of fluorescent carbon nanodots via cubosome lipid nanocarriers

Carbon nanodots (C-dots) have attracted much attention for their use in the fields of bioimaging, drug delivery, and sensing due to their excellent fluorescent and photoluminescent properties, photostability, biocompatibility, and amenability to surface modification. Herein, we report a nanocomposite formulation of C-dots (<5 nm) encapsulated in lipid-based lyotropic liquid crystalline nanoparticles (∼250 nm) via either passive diffusion or electrostatic mechanisms. The physicochemical properties of the nanocomposite formulation including particle size, surface charge, internal cubic nanostructures, and pH-dependent fluorescent properties were characterised. Upon loading of C-dots into lipid nanoparticles, the highly ordered inverse bicontinuous cubic mesophase existed in the internal phase of the nanoparticles, demonstrated by synchrotron small angle X-ray scattering, molecular dynamic simulation and cryogenic transmission electron microscopy. The pH-dependent fluorescent property of the C-dots was modified via electrostatic interaction between the C-dots and cationic lipid nanoparticles, which further enhanced the brightness of C-dots through self-quenching prevention. The cytotoxicity and cellular uptake efficiency of the developed nanocomposites were also examined in an epithelial gastric adenocarcinoma cell line (AGS) and a macrophage cell line (stimulated THP-1). Compared to free C-dots, the uptake and cell imaging potential of the C-dot nanocomposites was significantly improved, by several orders of magnitude as demonstrated by cytoplasmic fluorescent intensities using confocal microscopy. Loading C-dots into mesoporous lipid nanocarriers presents a new way of modifying C-dot physicochemical and fluorescent properties, alternative to direct chemical surface modification, and advances the bioimaging potential of C-dots by enhancing cellular uptake efficiency and converging C-dot light emission.

The role of molecular fluorophores in the photoluminescence of carbon dots derived from citric acid: current state-of-the-art and future perspectives

Carbon dots (CDs), an emerging class of nanomaterials, have attracted considerable attention due to their intriguing photophysical properties. Despite their indisputable potential of utilization in many fascinating areas of research and life, some fundamental aspects concerning their structure and the origin of their photoluminescence (PL) properties still await clarification. The mechanism of PL emission of CDs is associated with their structure, which is dependent on the carbonization process. At the initial stages of CD synthesis via a bottom-up approach, molecular fluorophores are considered to dominate the optical characteristics of the resulting nanomaterials. In this review, the recent progress in the use of molecular state theory for explanation of the structure-property relationship in CDs is summarized. This review focuses exclusively on the molecular fluorophores existing in nanomaterials prepared from citric acid (CA) as one of the most frequent carbon sources reported for the bottom-up synthesis of CDs. Consequently, the most relevant transformations of CA and the history of molecular fluorophores derived from it are described, followed by an in-depth discussion on their relevance in understanding the specific photophysical properties of blue-, green-, and red-emitting CDs. Finally, the challenging issues and future perspectives of molecular state PL mechanism exploration in CDs are highlighted.

Nitrogen-Doped Carbon Nanodots Produced by Femtosecond Laser Synthesis for Effective Fluorophores

Understanding the effect of heteroatom doping is crucial for the design of carbon nanodots (CNDs) with enhanced luminescent properties for fluorescence imaging and light-emitting devices. Here, we study the effect and mechanisms of luminescence enhancement through nitrogen doping in nanodots synthesized by the bottom-up route in an intense femtosecond laser field using the comparative analysis of CNDs obtained from benzene and pyridine. We demonstrate that laser irradiation of aromatic compounds produces hybrid nanoparticles consisting of a nanocrystalline core with a shell of surface-bonded aromatic rings. These nanoparticles exhibit excitation-dependent visible photoluminescence typical for CNDs. Incorporation of nitrogen into pyridine-derived CNDs enhances their luminescence characteristics through the formation of small pyridine-based fluorophores peripherally bonded to the nanoparticles. We identify oxidation of surface pyridine rings as a mechanism of formation of several distinct blue- and green-emitting fluorophores in nanodots, containing pyridine moieties. These findings shed additional light on the nature and formation mechanism of effective fluorophores in nitrogen-doped carbon nanodots produced by the bottom-up route.

Development of Fluorescent Carbon Nanoparticle-Based Probes for Intracellular pH and Hypochlorite Sensing

Acid-base and redox reactions are important mechanisms that affect the optical properties of fluorescent probes. Fluorescent carbon nanoparticles (CNPs) that possess tailored surface functionality enable a prompt response to regional stimuli, offering a useful platform for detection, sensing, and imaging. In this study, mPA CNPs were developed through one-pot hydrothermal reaction as a novel fluorescent probe (quantum yield = 10%) for pH and hypochlorite sensing. m-Phenylenediamine was chosen as the major component of CNPs for pH and hypochlorite responsiveness. Meanwhile, ascorbic acid with many oxygen-containing groups was introduced to generate favorable functionalities for improved water solubility and enhanced sensing response. Thus, the mPA CNPs could serve as a pH probe and a turn-off sensor toward hypochlorite at neutral pH through fluorescence change. The as-prepared mPA CNPs exhibited a linear fluorescence response over the pH ranges from pH 5.5 to 8.5 (R2 = 0.989), and over the concentration range of 0.125–1.25 μM for hypochlorite (R2 = 0.985). The detection limit (LOD) of hypochlorite was calculated to be 0.029 μM at neutral pH. The mPA CNPs were further applied to the cell imaging. The positively charged surface and nanoscale dimension of the mPA CNPs lead to their efficient intracellular delivery. The mPA CNPs were also successfully used for cell imaging and sensitive detection of hypochlorite as well as pH changes in biological systems. Given these desirable performances, the as-synthesized fluorescent mPA CNPs shows great potential as an optical probe for real-time pH and hypochlorite monitoring in living cells.

Rapid detection of silver ions based on luminescent carbon nanodots for multicolor patterning, smartphone sensors, and bioimaging applications

Photoluminescent nitrogen and sulfur co-doped carbon nanodots (N,S-CNDs) were prepared via single-step hydrothermal carbonization using 2,4-diaminobenzenesulfonic acid (2,4-DABSA) as the sole precursor. The synthesized N,S-CNDs are easily dispersed in aqueous solution and have an average particle size of 5.0 ± 0.2 nm, showing a high quantum yield of 23.1% with excellent stability. The surface states of the N,S-CNDs were confirmed by Fourier-transform infrared spectroscopy, powder X-ray diffractometry, Raman spectroscopy, and X-ray photoelectron spectroscopy techniques. These N,S-CNDs were applied for the rapid visual sensing detection of Ag⁺ ions, which can be identified by their photoluminescent color change under ultraviolet (UV) light illumination at 365 nm within 5 s. Furthermore, a linear correlation coefficient between P₀/P and Ag⁺ ions was observed in the linear range of 0-1.2 μM with a detection limit of 7.88 nM. The proposed method was successfully used for the sensitive detection of Ag⁺ ions in real samples with satisfactory recoveries and relative standard deviation. The photoluminescence properties of N,S-CND and N,S-CNDs/Ag⁺ aqueous solutions were demonstrated by their invisible inks that can only be seen when irradiated with UV light. The RGB values of N,S-CND and N,S-CNDs/Ag⁺ aqueous solutions were measured using a color selector smartphone application. In addition, N,S-CND and N,S-CNDs/Ag⁺ aqueous solutions were further used for the multicolor imaging of HCT-116 cancer cells due to the low toxicity of N,S-CNDs.

Application of high-efficiency green fluorescent carbon dots prepared by acid catalysis in multicolour LEDs

Acidic reagents play an important role in the preparation of carbon dots (CDs). Therefore, we prepared efficient green fluorescent CDs by potassium bisulfate, acetic acid and hydrochloric acid catalysis and discussed why the acid catalyst induced a fluorescence redshift and improved the quantum yield of the CDs. Furthermore, the concentration-dependent photoluminescence behaviour of the CDs was characterized. CD/PVA composites emitting green to yellow light were obtained by exploiting the fluorescence tunability of CDs. Based on different light-emitting diode substrates, green, yellow and white light-emitting diodes with excellent performance were prepared.