Plasmon-induced photoelectrochemistry in the visible region was studied at gold nanoparticle-nanoporous TiO(2) composites (Au-TiO(2)) prepared by photocatalytic deposition of gold in a porous TiO(2) film. Photoaction spectra for both the open-circuit potential and short-circuit current were in good agreement with the absorption spectrum of the gold nanoparticles in the TiO(2) film. The gold nanoparticles are photoexcited due to plasmon resonance, and charge separation is accomplished by the transfer of photoexcited electrons from the gold particle to the TiO(2) conduction band and the simultaneous transfer of compensative electrons from a donor in the solution to the gold particle. Besides its low-cost and facile preparation, a photovoltaic cell with the optimized electron mediator (Fe(2+/3+)) exhibits an optimum incident photon to current conversion efficiency (IPCE) of 26%. The Au-TiO(2) can photocatalytically oxidize ethanol and methanol at the expense of oxygen reduction under visible light; it is potentially applicable to a new class of photocatalysts and photovoltaic fuel cells.

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.

Nanoporous TiO(2) films loaded with gold and silver nanoparticles exhibit negative potential changes and anodic currents in response to visible light irradiation, so that the films would potentially be applicable to inexpensive photovoltaic cells, photocatalysts and simple plasmon sensors.

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.

A sensitive biosensor: A strategy for the intracellular imaging of Cu(2+) ions has been developed by integrating a recognition molecule, N-(2-aminoethyl)-N,N,N'tris(pyridin-2-ylmethyl)ethane-1,2-diamine (AE-TPEA), into a hybrid system composed of carbon and CdSe/ZnS quantum dots.

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.

The high conductive TiO(2) nanoneedles film is first employed as a support matrix for immobilizing model enzyme, cytochrome c (cyt c) to facilitate the electron transfer between redox enzymes and electrodes. Reversible and direct electron transfer of cyt c is successfully achieved at the nanostructured TiO(2) surface with the redox formal potential (E(0)') of 108.0 +/- 1.9 mV versus Ag|AgCl and heterogeneous electron transfer rate constant (k(s)) of 13.8 +/- 2.1 s(-1). Experimental data indicate that cyt c is stably immobilized onto the TiO(2) nanoneedles film and maintains inherent enzymatic activity toward H(2)O(2). On the basis of these results, the cyt c-TiO(2) nanocomposits film is capable of sensing H(2)O(2) at a suitable potential, 0.0 V (vs Ag|AgCl), where not only common anodic interferences like ascorbic acid, uric acid, 3,4-dihydroxyphenylacetic acid but also a cathodic interference, O(2), are effectively avoided. Besides high selectivity, the present biosensor for H(2)O(2) shows broad dynamic range and low detection limit. These remarkable analytical advantages, as well as the characteristic of TiO(2) nanoneedles film such as high conductivity, biocompatibility, and facile ability to miniaturize establishes a novel approach to detection of extracellular H(2)O(2) released from human liver cancer cells.

Seeing is believing: A direct, selective, and sensitive strategy for colorimetric visualization of cerebral dopamine has been developed for the first time using gold nanoparticles through the design of double molecular recognition (see picture). The simplicity of this method establishes a facile and reliable approach for monitoring cerebral species in brain chemistry, which may be related to physiological and pathological events.