Here, we propose a plasmonic enzyme-linked immunosorbent assay (ELISA) based on highly sensitive colorimetric detection of alkaline phosphatase (ALP), which is achieved by iodine-mediated etching of gold nanorods (AuNRs). Once the sandwich-type immunocomplex is formed, the ALP bound on the polystyrene microwells will hydrolyze ascorbic acid 2-phosphate into ascorbic acid. Subsequently, iodate is reduced to iodine, a moderate oxidant, which etches AuNRs from rod to sphere in shape. The shape change of AuNRs leads to a blue-shift of longitudinal localized surface plasmon resonance. As a result, the solution of AuNRs changes from blue to red. Benefiting from the highly sensitive detection of ALP, the proposed plasmonic ELISA has achieved an ultralow detection limit (100 pg/mL) for human immunoglobulin G (IgG). Importantly, the visual detection limit (3.0 ng/mL) allows the rapid differential diagnosis with the naked eye. The further detection of human IgG in fetal bovine serum indicates its applicability to the determination of low abundance protein in complex biological samples.

A B S T R A C TBased on enzymatic-like reaction mediated etching of gold nanorods (GNRs), an ultrasensitive visual method was developed for on-site detection of urine glucose. With the catalysis of MoO 4 2− , GNRs were efficiently etched by H 2 O 2 which was generated by glucose-glucose oxidase enzymatic reaction. The etching of GNRs lead to a blue-shift of logitudinal localized surface plasmon resonance of GNRs, accompanied by an obvious color change from blue to red. The peak-shift and the color change can be used for detection of glucose by the spectrophotometer and the naked eyes. Under optimal condition, an excellent sensitivity toward glucose is obtained with a detection limit of 0.1 μM and a visual detection limit of 3 μM in buffer solution. Benefiting from the high sensitivity, the successful colorimetric detection of glucose in original urine samples was achieved, which indicates the practical applicability to the on-site determination of urine glucose.

Here, we have carefully investigated iodine-mediated etching of gold nanorods (AuNRs) in the presence of iodate and applied this phenomenon to on-site detection of dissolved oxygen (DO). Under given conditions, the quantitative conversion of target analytes DO to iodine leads to the etching of AuNRs along the longitudinal direction with the aid of cetyltrimethylammonium. As a result, the longitudinal localized surface plasmon resonance shifts to a short wavelength. The peak-shift can be used for quantitative determination of DO and iodate by a spectrophotometer. The satisfactory results from DO detection in different water samples and iodate detection in table salt indicate the feasibility of the proposed methods. Moreover, the as-prepared colorimetric test paper would make the detection more economical and simpler.

A highly sensitive colorimetric metalloimmunoassay with a detection limit of 0.15 ng ml −1 for human IgG based on coppermediated etching of gold nanorods was proposed. The assay is more sensitive than traditional ELISA, electrochemical metalloimmunoassay and HRP mimic nanomaterial tag-based immunoassay.