Fluorescent carbon nanoparticles or carbon quantum dots (CQDs) are a new class of carbon nanomaterials that have emerged recently and have garnered much interest as potential competitors to conventional semiconductor quantum dots. In addition to their comparable optical properties, CQDs have the desired advantages of low toxicity, environmental friendliness low cost and simple synthetic routes. Moreover, surface passivation and functionalization of CQDs allow for the control of their physicochemical properties. Since their discovery, CQDs have found many applications in the fields of chemical sensing, biosensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis. This article reviews the progress in the research and development of CQDs with an emphasis on their synthesis, functionalization and technical applications along with some discussion on challenges and perspectives in this exciting and promising field.

Carbon quantum dots and their applications

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

New achievements in the realm of nanoscience and innovative techniques of nanomedicine have moved micro/nanoparticles (MNPs) to the point of becoming actually useful for practical applications in the near future. Various differences between the extracellular and intracellular environments of cancerous and normal cells and the particular characteristics of tumors such as physicochemical properties, neovasculature, elasticity, surface electrical charge, and pH have motivated the design and fabrication of inventive “smart” MNPs for stimulus-responsive controlled drug release. These novel MNPs can be tailored to be responsive to pH variations, redox potential, enzymatic activation, thermal gradients, magnetic fields, light, and ultrasound (US), or can even be responsive to dual or multi-combinations of different stimuli. This unparalleled capability has increased their importance as site-specific controlled drug delivery systems (DDSs) and has encouraged their rapid development in recent years. An in-depth understanding of the underlying mechanisms of these DDS approaches is expected to further contribute to this groundbreaking field of nanomedicine. Smart nanocarriers in the form of MNPs that can be triggered by internal or external stimulus are summarized and discussed in the present review, including pH-sensitive peptides and polymers, redox-responsive micelles and nanogels, thermo- or magnetic-responsive nanoparticles (NPs), mechanical- or electrical-responsive MNPs, light or ultrasound-sensitive particles, and multi-responsive MNPs including dual stimuli-sensitive nanosheets of graphene. This review highlights the recent advances of smart MNPs categorized according to their activation stimulus (physical, chemical, or biological) and looks forward to future pharmaceutical applications.

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Smart micro/nanoparticles in stimulus-responsive drug/gene delivery systems

Carbon and graphene quantum dots (CQDs and GQDs), known as zero-dimensional (0D) nanomaterials, have been attracting increasing attention in sensing and bioimaging. Their unique electronic, fluores...

Review of Carbon and Graphene Quantum Dots for Sensing

Fluorescent carbon dots (CDs) are a new class of carbon nanomaterials and have demonstrated excellent optical properties, good biocompatibility, great aqueous solubility, low cost, simple synthesis, etc. Since their discovery, various synthesis methods using different precursors have been developed, and are mainly classified as top-down and bottom-up approaches. For the mechanistic origin of CDs photoluminescence, three mechanisms have been proposed such as quantum confinement effect, surface state and molecule state. CDs have presented many applications, and this review article mainly focuses on the development of CDs based fluorescent sensors in recent 5 years. The sensing mechanisms, senor design and sensing properties to various targets are summarized. A broad range of analytes including cations, anions, small molecules, macromolecules, cells and bacteria have been discussed. In addition, the challenges and future directions for CDs as sensing materials are also presented.

Fluorescent carbon dots and their sensing applications

A robust solid sensing platform for an on-site operational and accurate detection of heavy metal is still a challenge. We introduce chitosan based carbon dots rooted agarose hydrogel film as a hybrid solid sensing platform for detection of heavy metal ions. The fabrication of the solid sensing platform is centered on simple electrostatic interaction between the NH3+ group present in the carbon dots and the OH– groups present in agarose. Simply on dipping the hydrogel film strip into the heavy metal ion solution, in particular Cr6+, Cu2+, Fe3+, Pb2+, Mn2+, the strip displays a color change, viz., Cr6+ → yellow, Cu2+ → blue, Fe3+ → brown, Pb2+ → white, Mn2+ → tan brown. The optical detection limit of the respective metal ion is found to be 1 pM for Cr6+, 0.5 μM for Cu2+, and 0.5 nM for Fe3+, Pb2+, and Mn2+ by studying the changes in UV–visible reflectance spectrum of the hydrogel film. Moreover, the hydrogel film finds applicability as an efficient filtration membrane for separation of these quintet heavy m...

Carbon Dots Rooted Agarose Hydrogel Hybrid Platform for Optical Detection and Separation of Heavy Metal Ions

Green chitosan-carbon dots nanocomposite hydrogel film with superior properties.

In this communication we report the preparation of fluorescent carbon dots (CDs) from chitosan gel. We also studied the photoluminescent (PL) properties of CDs prepared in different pH conditions. While CDs prepared at pH 3 give an intense sharp blue fluorescence, those prepared at pH 1 or 5 give a broad emission. In addition, we also investigated CDs prepared from chitosan/Ag and chitosan/Au nanocomposites. It was observed that although the emission was broad and less well defined, there was an enhancement of PL emission for the CDs prepared from chitosan/Ag or chitosan/Au nanocomposites.

Fluorescent carbon dots obtained from chitosan gel

Novel carbon dot coated alginate beads (CA-CD) exhibiting superior stability and swelling properties have been successfully prepared. CA-CD show exceptional stability in ambient condition and are stable at room atmosphere and temperature even after 60 days. Moreover, CA-CD show excessive swelling in comparison to calcium alginate (CA) beads. The beads were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and optical microscopy. The CA and CA-CD beads were investigated for their use as pH dependent sustained drug delivery vehicles taking tetracycline (TC) and tetracycline associated with β-cyclodextrin (β-TC) as model drug systems. It was observed that TC loading was 35% and 77% with CA and CA-CD, respectively. Tetracycline associated with β-cyclodextrin (β-TC) shows 48% loading for CA and much greater loading (as high as 90%) for CA-CD. At pH 1, CA-CD and CA beads show maximum drug release with TC cumulative release of 70% and 37% at 96 h, respectively. However, the delivery rates at pH 1 were slower in case of tetracycline associated with β-cyclodextrin (β-TC) loading showing 61% release for CA-CD and 22% for CA after 96 h. Thus, CA-CD can be suitably used as an effective drug delivery vehicle with maximum release obtained at pH 1 emphasizing its use in the gastrointestinal tract where pH is low. Also, the use of β-cyclodextrin with the drug as an inclusion complex renders the CA and CA-CD beads useful for slow and long-term drug administration.

Novel carbon dot coated alginate beads with superior stability, swelling and pH responsive drug delivery

We demonstrate carbon dot based sensor of catecholamine, namely, dopamine and ascorbic acid. Carbon dots (CDs) were prepared from a green source: commercially available Assam tea. The carbon dots prepared from tea had particle sizes of ∼0.8 nm and are fluorescent. Fluorescence of the carbon dots was found to be quenched in the presence of dopamine and ascorbic acid with greater sensitivity for dopamine. The minimum detectable limits were determined to be 33 μM and 98 μM for dopamine and ascorbic acid, respectively. The quenching constants determined from Stern-Volmer plot were determined to be 5 × 10−4 and 1 × 10−4 for dopamine and ascorbic acid, respectively. A probable mechanism of quenching has been discussed in the paper.

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Neelam Gogoi

Papers

Carbon Dots Rooted Agarose Hydrogel Hybrid Platform for Optical Detection and Separation of Heavy Metal Ions

Green chitosan-carbon dots nanocomposite hydrogel film with superior properties.

Fluorescent carbon dots obtained from chitosan gel

Novel carbon dot coated alginate beads with superior stability, swelling and pH responsive drug delivery

Carbon Dot Based Sensing of Dopamine and Ascorbic Acid