Nanoparticles are promising scaffolds for applications such as imaging, chemical sensors and biosensors, diagnostics, drug delivery, catalysis, energy, photonics, medicine, and more. Surface functionalization of nanoparticles introduces an additional dimension in controlling nanoparticle interfacial properties and provides an effective bridge to connect nanoparticles to biological systems. With fascinating photoluminescence properties, carbon dots (C-dots), carbon-containing nanoparticles that are attracting considerable attention as a new type of quantum dot, are becoming both an important class of imaging probes and a versatile platform for engineering multifunctional nanosensors. In order to transfer C-dots from proof-of-concept studies toward real world applications such as in vivo bioimaging and biosensing, careful design and engineering of C-dot probes is becoming increasingly important. A comprehensive knowledge of how C-dot surfaces with various properties behave is essential for engineering C-dots with useful imaging properties such as high quantum yield, stability, and low toxicity, and with desirable biosensing properties such as high selectivity, sensitivity, and accuracy. Several reviews in recent years have reported preparation methods and properties of C-dots and described their application in biosensors, catalysis, photovoltatic cells, and more. However, no one has yet systematically summarized the surface engineering of C-dots, nor the use of C-dots as fluorescent nanosensors or probes for in vivo imaging in cells, tissues, and living organisms. In this Account, we discuss the major design principles and criteria for engineering the surface functionality of C-dots for biological applications. These criteria include brightness, long-term stability, and good biocompatibility. We review recent developments in designing C-dot surfaces with various functionalities for use as nanosensors or as fluorescent probes with fascinating analytical performance, and we emphasize applications in bioimaging and biosensing in live cells, tissues, and animals. In addition, we highlight our work on the design and synthesis of a C-dot ratiometric biosensor for intracellular Cu(2+) detection, and a twophoton fluorescent probe for pH measurement in live cells and tissues. We conclude this Account by outlining future directions in engineering the functional surface of C-dots for a variety of in vivo imaging applications, including dots with combined targeting, imaging and therapeutic-delivery capabilities, or high-resolution multiplexed vascular imaging. With each application C-dots should open new horizons of multiplexed quantitative detection, high-resolution fluorescence imaging, and long-term, real-time monitoring of their target.

A carbon dot (C-Dot)-based two-photon fluorescent probe has been developed for the monitoring of pH changes across a broad range with high sensitivity and selectivity. The inorganic-organic probe also shows good biocompatibility and cell permeability, and thus can be successfully applied in bioimaging and biosensing of physiological pH in living cells, as well as living tissues at a depth of 65-185 μm.

Determination of hydroxyl radical ((•)OH) with high sensitivity and accuracy in live cells is a challenge for evaluating the role that (•)OH plays in the physiological and pathological processes. In this work, a ratiometric fluorescence biosensor for (•)OH was developed, in which gold nanocluster (AuNC) protected by bovine serum albumin was employed as a reference fluorophore and the organic molecule 2-[6-(4'-hydroxy)phenoxy-3H-xanthen-3-on-9-yl]benzoic acid (HPF) acted as both the response signal and specific recognition element for (•)OH. In the absence of (•)OH, only one emission peak at 637 nm ascribed to AuNCs was observed, because HPF was almost nonfluorescent. However, fluorescence emission at 515 nm attributed to the HPF product after reaction with (•)OH--dianionic fluorescein--gradually increased with the continuous addition of (•)OH, while the emission at 637 nm stays constant, resulting in a ratiometric determination of (•)OH. The developed fluorescent sensor exhibited high selectivity for (•)OH over other reactive oxygen species (ROS), reactive nitrogen species (RNS), metal ions, and other biological species, as well as high accuracy and sensitivity with low detection limit to ∼0.68 μM, which fulfills the requirements for detection of (•)OH in a biological system. In addition, the AuNC-based inorganic-organic probe showed long-term stability against light illumination and pH, good cell permeability, and low cytotoxicity. As a result, the present ratiometric sensor was successfully used for bioimaging and monitoring of (•)OH changes in live cells upon oxidative stress.

In this article, a ratiometric fluorescent biosensor for O2(•-) was developed, by employing carbon dots (C-Dots) as the reference fluorophore and hydroethidine (HE), a specific organic molecule toward O2(•-), playing the role as both specific recognition element and response signal. The hybrid fluorescent probe CD-HE only emitted at 525 nm is ascribed to C-Dots, while HE was almost nonfluorescent, upon excitation at 488 nm. However, after reaction with O2(•-), a new emission peak ascribed to the reaction products of HE and O2(•-) was clearly observed at 610 nm. Meanwhile, this peak gradually increased with the increasing concentration of O2(•-) but the emission peak at 525 nm stayed constant, leading to a ratiometric detection of O2(•-). The inorganic-organic fluorescent sensor exhibited high sensitivity, a broad dynamic linear range of ~5 × 10(-7)-1.4 × 10(-4) M, and low detection limit down to 100 nM. The present probe also showed high accuracy and excellent selectivity for O2(•-) over other reactive oxygen species (ROS), metal ions, and so on. Moreover, the C-Dot-based inorganic-organic probe demonstrated long-term stability against pH changes and continuous light illumination, good cell-permeability, and low cytotoxicity. Accordingly, the developed fluorescent biosensor was eventually applied for intracellular bioimaging and biosensing of O2(•-) changes upon oxidative stress.

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