The synthetic strategies, photoluminescence mechanisms and promising applications of carbon dots: Current state and future perspective

Carbon dots (CDs), comprising crystalline graphitized carbon cores and polymer surface groups, are currently attracting a lot of interest in biological fields owing to their fluorescent properties, high photostability, biocompatibility and low toxicity. In addition, the easy preparation and functionalization of CDs stimulate the development of CDs‐based composite materials with specific functions. Presently, the biological applications of CDs are growing at a remarkable speed, justifying the need for up‐to‐date review articles that capture recent progress in this blossoming field. In this review, breakthroughs in the synthesis, modification, optical properties, toxicology and biocatalytic platforms of CDs are described. Further, recent research related to bioimaging, biosensing, drug delivery, antibacterial, anticancer (photothermal therapy, photodynamic therapy and synergistic therapy) and antiviral therapies involving CDs are discussed in detail. Finally, a perspective on the prospects and challenges of CDs in the fields of biomedicine and biotechnology is provided.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/smsc.202200012

Carbon Dots in Bioimaging, Biosensing and Therapeutics: A Comprehensive Review

Advances in nanotechnology and nanomaterials have attracted considerable interest and play key roles in scientific innovations in diverse fields. In particular, increased attention has been focused on carbon-based nanomaterials exhibiting diverse extended structures and unique properties. Among these materials, zero-dimensional structures, including fullerenes, carbon nano-onions, carbon nanodiamonds, and carbon dots, possess excellent bioaffinities and superior fluorescence properties that make these structures suitable for application to environmental and biological sensing, imaging, and therapeutics. This review provides a systematic overview of the classification and structural properties, design principles and preparation methods, and optical properties and sensing applications of zero-dimensional carbon nanomaterials. Recent interesting breakthroughs in the sensitive and selective sensing and imaging of heavy metal pollutants, hazardous substances, and bioactive molecules as well as applications in information encryption, super-resolution and photoacoustic imaging, and phototherapy and nanomedicine delivery are the main focus of this review. Finally, future challenges and prospects of these materials are highlighted and envisaged. This review presents a comprehensive basis and directions for designing, developing, and applying fascinating fluorescent sensors fabricated based on zero-dimensional carbon nanomaterials for specific requirements in numerous research fields.

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging.

It is challenging to realize ultra-pH-responsive fluorescent carbon dots (CDs), especially reversibly adjustable multichromatic emission covering a broad wavelength range including deep-red lasing. This study reports a one-step solvothermal treatment of pyrogallol to obtain brilliant CDs with a substantial bathochromic shift (from blue to dark red emission) induced by pH changes. The CDs show an excellent reversible pH response: simple acid treatment instantaneously shifts their initial green emission by 140 nm to a deep-red emission with high-color-purity and enhances brightness, subsequent alkali addition returned the green emission, suggesting smooth and reversible regulation of fluorescence. Detailed components analyses, morphology tracking, and photodynamic studies demonstrate the discrete, reversible shift of emission derived from structural transformations, which include protonation of the CDs and their subsequent assembly and partial aggregation triggered by hydrogen bonding. The abundant H-bonds crosslink the network of CDs, ultimately raising the quantum yield from 11.3% to 16.0%. Furthermore, the aggregated CDs crystallized into sheet-like structures that acted as a resonant cavity for lasing show excellent gain ability and lead to strong deep-red lasing. This study draws together pH regulation, the effects of CD's assembly, and their lasing emission to demonstrate significant potential for the development of versatile CDs. 

Acid‐Triggered Aggregation of Carbon Dots Shifted Their Emission to Give Unexpected Deep‐Red Lasing

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 reading"encryption technology a reality.

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

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