Zhiqin Yuan

Fluorescent Gold Nanoclusters: Recent Advances in Sensing and Imaging

F gold nanoclusters (Au NCs) or nanodots (NDs) with sizes smaller than 3 nm are a specific type of gold nanomaterials. In this review, Au NCs are used to represent fluorescent Au nanomaterials with sizes smaller than 3 nm. Unlike the most popular and well-known spherical, large gold nanoparticles, Au NCs do not exhibit surface plasmon resonance (SPR) absorption in the visible region but have fluorescence in the visible to near-infrared (NIR) region. With advantages of long lifetime, large Stokes shift, and biocompatibility, Au NCs have become interesting sensing and imaging materials. Although Au NCs prepared from Au in the presence of small thiol compounds such as 2-phenylethanethiol (PhCH2CH2SH) have been reported over the past decade, 5 their use for bioapplications have not been well recognized, mainly because of their low quantum yield (usually less than 1%), poor water dispersibility, photo and chemical instability, and difficulty for conjugation. In the past decade, many strategies for the preparation of stable, water dispersible, highly fluorescent, and biocompatible Au NCs have been reported. There are two major categories elucidating the recent advanced techniques for the preparation of Au NCs. The first category is through etching of larger sizes of nonfluorescent gold nanoparticles (Au NPs) by thiol compounds such as mercaptopropionic acid. The second category is from reduction of Au in the presence of a ligand or template (protein) such as bovine serum albumin (BSA). The optical properties of biocompatible Au NCs are dependent on their size, surface ligand or template, and the surrounding medium, and thus they can be studied to develop sensitive and selective sensing and imaging systems for the detection of various analytes. The growing popularity of Au NCs in analytical applications has been realized in these few years. Several excellent review papers dealing with Au NCs from the viewpoint of analytical chemistry have been reported to highlight their potential for the analysis of environmental and biological samples. This review focuses on recent advances in Au NCs based sensing and imaging systems between 2012 and 2014. Current challenges and future prospects of Au NCs for fundamental studies and analytical applications will be provided.

Sensitive and Selective Detection of Copper Ions with Highly Stable Polyethyleneimine-Protected Silver Nanoclusters

Copper is a highly toxic environmental pollutant with bioaccumulative properties. Therefore, sensitive Cu(2+) detection is very important to prevent over-ingestion, and visual detection using unaugmented vision is preferred for practical applications. In this study, hyperbranched polyethyleneimine-protected silver nanoclusters (hPEI-AgNCs) were successfully synthesized using a facile, one-pot reaction under mild conditions. The hPEI-AgNCs were very stable against extreme pH, ionic strength, temperature, and photoillumination and could act as sensitive and selective Cu(2+) sensing nanoprobes in aqueous solutions with a 10 nM limit of detection. In addition, hPEI-AgNCs-doped agarose hydrogels were developed as an instrument-free and regenerable platform for visual Cu(2+) and water quality monitoring.

Functionalized fluorescent gold nanodots: synthesis and application for Pb2+ sensing

We developed a novel strategy to prepare functionalized fluorescent gold nanodots (AuNDs) based on a ligand exchange reaction and demonstrated that glutathione modified AuNDs can be utilized for highly sensitive and selective Pb(2+) sensing in aqueous solution.

Selective Colorimetric Detection of Hydrogen Sulfide Based on Primary Amine-Active Ester Cross-Linking of Gold Nanoparticles

Hydrogen sulfide (H2S) is a highly toxic environmental pollutant and also an important gaseous transmitter. Therefore, selective detection of H2S is very important, and visual detection of it with the naked eye is preferred in practical applications. In this study, thiolated azido derivates and active esters functionalized gold nanoparticles (AE-AuNPs)-based nanosensors have been successfully prepared for H2S perception. The sensing principle consists of two steps: first, H2S reduces the azide group to a primary amine; second, a cross-linking reaction between the primary amine and active ester induces the aggregation of AuNPs. The AE-AuNPs-based nanosensors show high selectivity toward H2S over other anions and thiols due to the specific azide-H2S chemistry. Under optimal conditions, 0.2 μM H2S is detectable using a UV-vis spectrophotometer, and 4 μM H2S can be easily detected by the naked eye. In addition, the practical application of the designed nanosensors was evaluated with lake water samples.

Color Difference Amplification between Gold Nanoparticles in Colorimetric Analysis with Actively Controlled Multiband Illumination

Spectral chemical sensing with digital color analysis by using consumer imaging devices could potentially revolutionize personalized healthcare. However, samples with small spectral variations often cannot be differentiated in color due to the nonlinearity of color appearance. In this study, we address this problem by exploiting the color image formation mechanism in digital photography. A close examination of the color image processing pipeline emphasizes that although the color can be represented digitally, it is still a reproducible subjective perception rather than a measurable physical property. That makes it possible to physically manage the color appearance of a nonradiative specimen through engineered illumination. By using scattering light imaging of gold nanoparticles (GNPs) as a model system, we demonstrated via simulation that enlarged color difference between spectrally close samples could be achieved with actively controlled illumination of multiple narrow-band light sources. Experimentally, darkfield imaging results indicate that color separation of single GNPs with various sizes can be significantly improved and the detection limit of GNP aggregation-based colorimetric assays can be much reduced when the conventional spectrally continuous white light was replaced with three independently intensity-controlled laser beams, even though the laser lines were uncorrelated with the LSPR maxima of the GNPs. With low-cost narrow-band light sources widely available today, this actively controlled illumination strategy could be utilized to replace the spectrometer in many spectral sensing applications.

Silver nanoclusters as fluorescent nanosensors for selective and sensitive nitrite detection

As a highly carcinogenic pollutant but also an important food preservative, nitrite has attracted growing attention in the past few years. In this work, a hyperbranched polyethyleneimine protected silver nanocluster (hPEI-Ag NC) based nanosensor is proposed for the detection of nitrite. The sensing principle consists of two steps: nitrite reacts with hydrogen peroxide (H2O2) to generate peroxynitrous acid under acidic conditions, and the as-formed peroxynitrous acid then induces the aggregation and fluorescence quenching of hPEI-Ag NCs. Due to the specific nitrite–H2O2 chemistry, the hPEI-Ag NC based nanosensor shows high selectivity toward nitrite over other anions with a limit of detection of 100 nM. The practical application of the proposed nanosensor has been validated with lake water, tap water and seawater samples.

Photoluminescent gold nanodots: role of the accessing ligands

In this study, we prepared a series of photoluminescent gold nanodots (Au NDs) from gold nanoparticles (∼3 nm) via etching with 11-mercaptoundecanoic acid (11-MUA) under various ionic strengths or various co-accessing alkanethiol (RSH) ligands. We have demonstrated that self-assembly of 11-MUA molecules on Au NDs under various salt concentrations, and co-immobilization of (11-mercaptoundecyl)trimethylammonium bromide/11-MUA or α-lipoic acid/11-MUA on Au NDs play significant roles in determining particle sizes and thus their optical properties, such as quantum yield, absorption and emission wavelength, and emission lifetime. Our findings reveal that the photoluminescence of Au NDs depends on the ligand density, chain length and electron donation of alkanethiols, and the core size.

Rapid Screening of Oxygen States in Carbon Quantum Dots by Chemiluminescence Probe

The oxygen states (O-states) of carbon quantum dots (CDs) play an important role, with regard to their optical properties and analytical applications. However, the rapid screening of O-states in CDs is still a great challenge, because of the complicated surface composition. In this study, it is found that the chemiluminescence (CL) intensity of prepared CDs in the presence of peroxynitrite (ONOO-) is proportional to the content of C-O group-related O-states. The related mechanism discloses that the O-state-dependent CL is due to the fact that abundant C-O functional groups in CDs with high O-states could facilitate the electron transfer of the produced smaller energy gaps for strong CL emission. Hence, ONOO--induced CL can be utilized as a facile probe for the rapid screening of O-states in CDs with some advantages, such as rapid response, low cost, and easy operation. Its practicability is verified by detecting the CL of phosphorus-doped CDs with variable phosphorus-doping contents. The content of C-O group-related O-states in sulfur-/phosphorus-doped CDs measured by the proposed CL probe is consistent with X-ray photoelectron spectroscopy (XPS) characterization. This strategy can also be extended to distinguish O-states in different types of nanoparticles by tuning the CL probe molecules.

Layered-nanomaterial-amplified chemiluminescence systems and their analytical applications

AbstractLayered nanomaterial has become a popular hierarchical material for amplifying chemiluminescence (CL) in recent years, mainly because of its ease of preparation and modification, large specific surface area, and high catalytic activity. In this review, we mainly discuss layered-nanomaterial-amplified CL systems based on graphene and its derivatives, layered double hydroxides, and clay. Detection mechanisms and strategies of layered-nanomaterial-amplified CL systems are provided to show the basic concepts for designing sensitive and selective sensing systems. Strategies for expanding the applications of layered-nanomaterial-amplified CL systems by combination with surfactants, quantum dots, organic dyes, and nanoparticles are introduced for the analysis of various analytes in real samples. The challenges and future trends of layered-nanomaterial-amplified CL systems are discussed at the end of the review. Graphical AbstractSchematic illustration of layered nanomaterial amplified chemiluminescence

Hyperbranched polyamine assisted synthesis of dual-luminescent gold composite with pH responsive character

We present a facile one-pot, two-step strategy to prepare water-soluble dual-luminescent gold nanodots (AuNDs) and few-atom gold nanocluster composites simultaneously by using high molecular weight and hyperbranched polyethyleneimine (hPEI) as the protection ligand and stabilization agent. It was found that in the presence of hPEI, Au(III) ion can be reduced to a metastable Au(I) charge state in aqueous solution. Subsequently, adding 11-mercaptoundonioic acid induces parallel pathways of restricted Au(I) assembly, leading to the formation of both red-emitting hPEI stabilized AuNDs and blue-emitting hPEI-protected Au8 nanoclusters. The intensity ratio between the blue and red species shows a sensitive and reversible response to the solution pH in the range 2-11 and the dual-luminescent gold composites can act as an effective and reversible pH indicator.