In this study, we synthesized novel gold-carbon dots (GCDs) with unique properties by microwave-assisted method. The characterization of high-resolution transmission electron microscope (HRTEM), XRD, high-angle annular dark field scanning transmission electron microscope (HAADF-STEM), and energy dispersive spectrometer demonstrates that GCDs are composed of carbon and Au. Tiny Au clusters are dispersed in a 2 nm-size carbon skeleton, which integrates the properties of typical CDs and gold nanoclusters (AuNCs), displaying fascinating peroxidase-like activity and single excitation/dual emission. Dual emission of the GCDs exhibits different fluorescent response to the target species and enables the GCDs to be exploited for sensing and bioimaging. The highly photostable and biocompatible GCDs were applied to dual fluorescent imaging for breast cancer cells and normal rat osteoblast cells under a single excitation. Moreover, ratiometric fluorescence imaging was used to monitor Fe(3+) level in normal rat osteoblast cells.

Ultra-sensitive colorimetric determination of H2O2 is accomplished based on the intrinsic peroxidase-like activity of Au nanoclusters (AuNCs) stabilized by glutathione (GSH). The color change of 3,3,5,5-tetramethylbenzidine (TMB) catalyzed by AuNCs offers an indirect method to measure glucose. This sensing platform makes use of a dual optical signal change, including the color change in an aqueous solution under visible light illumination and an ultra-sensitive fluorescent assay arising from efficient fluorescence resonance energy transfer (FRET) between the AuNCs and oxidized TMB. The detection limits of H2O2 and glucose are 4.9 × 10(-13) M and 1.0 × 10(-11) M, respectively. In addition, enhanced fluorescence is observed from the AuNCs due to the use of ethanol which produces clear changes in the quantum yield and lifetime of the AuNCs. The quantum yield of AuNCs is enhanced from ∼12.5% as an isolated fluorophore to 38.9% in an AuNCs-ethanol complex. The enhanced fluorescence lowers the detection limits of H2O2 and glucose by 2 orders of magnitude compared to those attained from the original AuNCs.

It is broadly interesting but remains a big challenge to explore nanomaterials-based methods to enable naked-eye observation and determination of ultratrace biomarkers and drugs. In this study, we developed a straightforward and extendable plasmonic nanosensor to enable visually quantitative determination of ultratrace target molecules through combining the use of enzyme-mimetic gold nanoclusters (AuNCs). Starting from sandwiched antibody-antigen (i.e., an analyte)-antibody structure, we conjugated AuNCs on the outer layer antibody to catalyze the decomposition of hydrogen peroxide used to reduce HAuCl4 into gold nanopartilces (AuNPs) for naked eye readout. This strategy is in theory applicable to all immunoreactions available and the protocol proposed to attach AuNCs onto an antibody is suitable to all proteins. The applicability of this type of nanosensor was validated by the determination of various ultratrace analytes such as protein avidin, breast cancer antigen, thyroid hormone, and even methamphetamine (MA), giving a naked-eye-readout limit of detection (LOD), down to 1.0 × 10(-20) M protein avidin, 7.52 × 10(-14) U/mL breast cancer antigen 15-3, 2.0 × 10(-15) mg/mL 3,5,3'-L-triiodothyronine and 2.3 × 10(-18) mg/mL MA. This strategy is thus considered an ultrasensitive way to fabricate plasmonic nanosensors, having wide and invaluable application potential in clinical, biological, and environmental studies, and in food quality control.

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