pH-Elicited Luminescence Functionalities of Carbon Dots: Mechanistic Insights

Insight into the photophysics of strong dual emission (blue & green) producing graphene quantum dot clusters and their application towards selective and sensitive detection of trace level Fe3+ and Cr6+ ions

Graphene-nanostructured systems, such as graphene quantum dots (GQDs), are well known for their interesting light-emitting characteristics and are being applied to a variety of luminescence-based applications. The emission properties of GQDs are complex. Therefore, understanding the science of the photophysics of coupled quantum systems (like quantum clusters) is still challenging. In this regard, we have successfully prepared two different types of GQD clusters, and explored their photophysical properties in detail. By co-relating the structure and photophysics, it was possible to understand the emission behavior of the cluster in detail. This gave new insight into understanding the clustering effect on the emission behaviour. The results clearly indicated that although GQDs are well connected, the local discontinuity in the structure prohibits the dynamics of photoexcited charge carriers going from one domain to another. Therefore, an excitation-sensitive dual emission was possible. Emission yield values of about 18% each were recorded at the blue and green emission wavelengths at a particular excitation energy. This meant that the choice of emission color was decided by the excitation energy. Through systematic analysis, it was found that both intrinsic and extrinsic effects contributed to the blue emission, whereas only the intrinsic effect contributed to the green emission. These excitation-sensitive dual emissive GQD clusters were then used to sense Fe³⁺ and Cr⁶⁺ ions in the nanomolar range. While the Cr⁶⁺ ions were able to quench both blue and green emissions, the Fe³⁺ ions quenched blue emission only. The insensitivity of the Fe³⁺ ions in the quenching of the green emission was also understood through quantum chemical calculations.

Schematic representation for the origin of blue and green emissions, and the resultant PL emission spectra from the GQD interconnected cluster-type sample.

Carbon dot-based composites for catalytic applications

We summarize the construction methods and influencing factors of CDs-based composites and discuss their catalytic applications, including photocatalysis, chemical catalysis, peroxidase-like catalysis, Fenton-like catalysis and electrocatalysis.

Label-Free Pathogen Detection Based on Yttrium-Doped Carbon Nanoparticles up to Single-Cell Resolution

The capability to detect bacteria at a low cell density is critical to prevent the delay in therapeutic intervention and to avoid the emergence of antibiotic-resistant species. Till date, significant advancement has been made to develop a sensing platform for rapid and reliable bacterial detection. However, critical requirements, that is, limit of detection, fast time of response, ultrasensitivity with high reproducibility, and the ability to distinguish between bacterial strains are yet to be met within a single sensing platform. In this contribution, we present a novel label-free sensor based on pH-sensitive fluorescent yttrium-doped carbon nanoparticles (YCNPs) embedded in agarose that can rapidly and accurately detect and discriminate pathogens in real time. The developed sensor matrix presented pH-triggered aggregation-induced emission quenching of YCNPs in a wide pH range. When the pH decreased from 10.0 to 4.0, the fluorescence of the matrix decreased linearly (R² = 0.9229). The sensor 's high sensitivity in a physiologically relevant pH range enables the monitoring of the presence of live pathogens to single-cell resolution. In addition, the 3D matrix sensor showed low cytotoxicity and long stability (>30 days). Besides, the YCNP platform is stable for several hours (5 h) in a complex medium and does not alter the bacterial activities, allowing real-time monitoring of bacterial growth with a small sample volume (100 μL) and rapid response time (25 min). Furthermore, using machine learning-assisted tools, different bacterial strains with various cell densities were discriminated with an accuracy of almost 100%. Moreover, blends of pathogens and a real-world sample can also be identified accurately, thus enabling the sensor to provide fast and reliable pathogen information for clinical decisions and allowing continuous monitoring of infectious disease trends.

Carbonized Polymer Dots: A Brand New Perspective to Recognize Luminescent Carbon-Based Nanomaterials

Carbon dots (CDs), as emerging luminescent nanomaterials, possess excellent but complex properties, bringing about extensive attention and a lot of controversy. In this Perspective, we put forward the concept of "carbonized polymer dots" and emphasize the important role of polymerization and carbonization during the formation of CDs. We explore the common characters and clarify the complicated relationship of CDs, based on the reasonable classification of graphene quantum dots, carbon quantum dots, and carbonized polymer dots. Moreover, different perspectives are provided for comprehensive analysis about the essence of CDs, including quantum dots, molecules, and polymers. The photoluminescence mechanism has been classified into molecule state, carbon core state, surface/edge state, and cross-link enhanced emission effect for further understanding of complicated phenomena.

Solvent Effects: A Signature of J- and H-Aggregate of Carbon Nanodots in Polar Solvents

The secret behind excitation-dependent/-independent photoluminescence of carbon nanodots (CDs) is not yet revealed completely. To address this issue, a detailed investigation on solvent polarity-dependent optical properties of citric acid-urea co-derived nitrogen-doped carbon nanodots (NCDs) was carried out. The interpretation on UV-visible spectral data reveals the presence of H-aggregates formed through hydrogen bonding. In addition, dipole-dipole interaction-mediated J-aggregates are clearly evident. The broad and intense excitation band of NCDs is mostly contributed by highly emissive J-like self-assembly of H-aggregates in polar solvents. Time-resolved fluorescence spectra of NCDs show triexponential decay kinetics. The three lifetime components correspond to long-lived H-aggregates, short-lived J-aggregates, and JH-aggregates of intermediate lifetime. Moreover, fluorescence of NCD is influenced by concentration and storage time. Accordingly, mismatch in spectral shapes of excitation and absorption spectra of NCD can be successfully correlated to aggregate species of NCDs that exist even in very dilute solutions.

Simultaneous and Reversible Triggering of the Phase Transfer and Luminescence Change of Amidine-Modified Carbon Dots by CO2

The ability to reversibly manipulate the surface nature of luminescent nanoparticles upon external stimulation enables the development of advanced optical probes for biological sensing and data encoding. Herein, we report the synthesis of a new class of smart carbon dots (CDs) via surface modification of amine-enriched CDs with CO₂-responsive groups of amidine. We present that alternative CO₂ and N₂ bubbling can not only lead to a reversible phase transfer of the CDs between an organic phase and an aqueous phase but also give rise to a corresponding reversible luminescence change between blue and cyan-green. We attribute these observations to changes in both the surface chemistry and the emission states of the CDs triggered by the alternative CO₂/N₂ introduction. We also find a similar luminescence change of the CDs upon alternative exposure to a humid vapor of CO₂ and a mixture of NH₃ and N₂ at room temperature, allowing them to be used as a new class of optical materials for optical encoding.

Carbon dots derived from algae as H2O2 sensors: the importance of nutrients in biomass

Carbon dots produced hydrothermally from algae were used directly for H2O2 sensing. The mineral nutrients in biomass were found be important for the composition, crystallinity, dispersion and photoluminescence (PL) quenching of the carbon dots under reactive oxygen species, which catalysed the oxidation of passivating ligands.

Photophysics and luminescence quenching of carbon dots derived from lemon juice and glycerol

During the past decade, carbon dots have emerged as a fascinating class of luminescent nanomaterials with versatile application potentials in bioimaging, labeling, photocatalysis and optoelectronics. Currently, intensive research is concentrated on understanding the intriguing optical properties of these promising materials and their utility as luminescence sensors. In this article, we describe the photoluminescence of carbon dots obtained from a bioresource (lemon juice) and from a small molecule precursor (glycerol), especially the quenching of their emission by nitrobenzene and Hg²⁺ ions, as representative cases. Stern-Volmer analysis using steady-state and time-resolved emission measurements, suggests the involvement of both transient quenching and dynamic quenching mechanisms in the interaction of the carbon dots with nitrobenzene. The radius of the quenching sphere is estimated to be slightly greater than the contact distances between the respective carbon dots and nitrobenzene, which is in reasonable agreement with the "sphere of action" model for transient quenching. In the interaction with Hg²⁺ ions, electrostatic attraction plays a major role, and the quenching mechanism involves predominantly static and dynamic quenching. The static quenching constant matches well with the binding constant of the carbon dots with the metal ion.

K. P, Pal H, Dutta Choudhury S. An insight into the molecular and surface state photoluminescence of carbon dots revealed through solvent-induced modulations in their excitation wavelength dependent emission properties

Solvent environment can uniquely alter excitation wavelength dependent photoluminescence of carbon dots.

Miscanthus grass-derived carbon dots to selectively detect Fe3+ ions

Novel fluorescent carbon dots (CDs) were synthesized using an economically feasible and green one-step heating process. Miscanthus, a perennial grass and an inexpensive sustainable biomass, was utilized as the starting material to prepare CDs and doped CDs (nitrogen, phosphorous and nitrogen-phosphorous dual doped). The abundance of oxygen-containing functional groups in Miscanthus-derived CDs (MCD) and doped MCD was confirmed via Fourier-transform infrared (FTIR) and energy dispersive X-ray spectroscopy (EDS). The average size of MCD, N-doped MCD, P-doped MCD and dual-doped MCDs was found to be 7.87 ± 0.27, 4.6 ± 0.21, 6.7 ± 0.38 and 5.3 ± 0.32 nm, respectively. The synthesized MCD and doped MCD exhibited a quantum yield (QY) of 4.71, 11.65, 2.33 and 9.63% for the MCD, N-doped MCD, P-doped MCD and dual-doped MCD, respectively. MCD and doped MCD exhibited excellent excitation-dependent photoluminescence properties, with strong blue fluorescence upon irradiation with UV-light (365 nm). N-doped MCD exhibited superb selectivity towards Fe³⁺ ions, with a detection limit of 20 nM and a detection range from 0.02 to 2000 μM. The normalized linear relationship between the intensity of fluorescence emission of the prepared N-doped MCD and the concentration of Fe³⁺ ions was utilized to selectively and sensitively detect Fe³⁺ ions.

Fluorescent carbon dots for the selective and sensitive detection of Fe³⁺ ions with a wide detection range and very low detection limit.

Pineapple Peel-Derived Carbon Dots: Applications as Sensor, Molecular Keypad Lock, and Memory Device

Herein, the fluorescent carbon dots (CDs) with blue emission were prepared by hydrothermal treatment using pineapple peel as a source of carbon. The as-prepared CDs exhibited turn-Off fluorescence behavior toward Hg2+ and subsequent turn-On behavior for l-cysteine along with enhanced biocompatibility and negligible cytotoxicity for cell imaging. The practical applicability of carbon dots was used for the quantification of Hg2+ in water. On the basis of the spectral characteristic changes, we have designed individual elementary logic operations such as NOT and IMP gates, by utilizing CD as probe and Hg2+ and l-Cys as chemical inputs. We have also demonstrated the utility of this system in electronic security devices and as memory element, with the idea of the switching.

Green Synthesized Carbon Quantum Dots from Polianthes tuberose L. Petals for Copper (II) and Iron (II) Detection

In this work carbon quantum dots (CQDs) are synthesized via a simple, low cost and as well as green way using tuberose (Polianthes tuberose L.) petals as the carbon source for the first time. We have not done any surface modification to the prepared CQDs although we directly employed this as fluorescent probe for the sensitive and selective detection of Fe²⁺ and Cu²⁺ ions. Both these ions drastically quench the emission intensity of the CQDs; in case of Cu²⁺ ions quenched CQDs EDTA results in regaining the fluorescence property but for Fe²⁺ ions quenched CQDs no such effect of EDTA is found. The limit of detection (LOD) is observed to be 200 nM in case of Cu²⁺ which is much lower than the safe limit provided by the WHO in drinking water. Hence the CQDs prepared in this simple and low cost method may find an important role in monitoring the water quality. The quantum yield of the CQDs prepared in our method is around 3%. Transmission electron microscope shows picture of nicely shaped CQDs with average size ~ 4 nm.

A chemical/molecular 4-input/2-output keypad lock with easy resettability based on red-emission carbon dots-Prussian blue composite film electrodes

In this work, a resettable 4-input/2-output keypad lock system based on red-emission carbon dots (rCDs) and Prussian blue (PB) modified electrodes was developed. Electrochromic PB layers were first electrochemically deposited on the indium tin oxide (ITO) electrode surface. An admixture of rCDs and chitosan (Chi) was then cast on the surface of PB layers, forming rCDs-Chi/PB film electrodes. UV-vis absorption of the films was sensitive to the applied potential since the blue PB constituent of the films would be transformed to nearly colorless Prussian white (PW) at the reduction potential of -0.2 V and then from PW back to PB at the oxidation potential of 0.4 V, and the transformation between PB and PW would also influence the fluorescence emission of the rCD constituent in the films. The addition of cysteine (Cys) in the testing solution could reduce the PB in the films into PW and generate an amperometric electrocatalytic current at 0.4 V. Meanwhile, the addition of Fe3+ in solution could greatly quench the fluorescence from the rCD component in the films. Thus, the responses of UV-vis absorbance, fluorescence emission and amperometric current of the rCDs-Chi/PB film electrode system exhibited potential-, Cys- and Fe3+-responsive switching properties. Based on the aforementioned work, a combinational logic gate circuit with 3 inputs and 3 outputs was established. In particular, on the same platform, a novel chemical/molecular 4-input/2-output keypad lock with easy resettability was elaborately designed with amperometric current and fluorescence peak intensity as two different types of outputs, so that a higher security level could be achieved.

Carbon nanodot-induced gelation of a histidine-based amphiphile: application as a fluorescent ink, and modulation of gel stiffness

This study demonstrates carbon dots induced hydrogelation of an amino acid based amphiphile and the potential use of this gel as a fluorescent ink.