Here, we report a simple visual colorimetric sensing system based on the self-assembly of gold nanorods (AuNRs) and graphene oxide (GO) for heparin detection using a polycationic polymer, polydiallyldimethylammonium chloride (PDADMAC), as a molecular probe. Within the system, the polycationic PDADMAC polymer binds tightly with GO, preventing the absorption of AuNRs on the GO surface. Consequently, the AuNRs are well dispersed in the system and maintain their native red color. In the presence of heparin, PDADMAC forms a stable complex with heparin, rendering the AuNRs self-assembled on the GO surface. As a result, this leads to AuNR aggregation and a change in color from red to purple due to a decrease in localized surface plasmon resonance (LSPR). The change in the absorption maxima of the AuNRs upon the presence of heparin was recorded. Under the optimized conditions, heparin can be detected in the range of 20–140 ng mL⁻¹ (R² = 0.9524). Using 3σ/s, where σ is the standard deviation of the blank and s is the slope of the linear graph, heparin can be detected at as low as 10.4 nanograms with a sample volume of 100 μL. The system also demonstrates selectivity of heparin detection against other potential interferences including ions and proteins.

Here, we report a simple colorimetric detection method for the determination of protamine based on the self-assembly of gold nanorods (AuNRs) on graphene oxide (GO). Within the system, the positively charged AuNRs could adsorb electrostatically on negatively charged GO, resulting in AuNR aggregation and color change from red to dark purple. Upon the addition of protamine, AuNRs were freely dispersed and remained red in color due to the strong adsorption between GO and protamine, preventing AuNRs from aggregation. The formation of GO–AuNR complexes and dispersed AuNRs was confirmed by transmission electron microscopy. The degree to which AuNRs aggregated was inversely proportional to the amount of protamine present in the system. The change in the absorption maxima of AuNRs in the presence of protamine was monitored. Using this simple system, under optimized conditions, protamine could be detected in the range of 100–700 ng mL⁻¹ (R² = 0.9683). Based on 3σ/s, where σ is the standard deviation of blank and s is the slope of the linear graph, the system could detect protamine as low as 6.3 ng with the sample volume of 100 μL. The system also demonstrated good selectivity towards interferences from other related physiological molecules including human serum albumin, hemoglobin protein, glucose, essential ion species, and other positively charged drugs.

A simple fluorescence-based lateral flow test platform for rapid influenza B virus screening as a model target molecule was successfully developed. In this work, Cy5-loaded silica nanoparticles were directly conjugated to monoclonal antibodies, specific to the influenza B nucleoprotein, via a direct physisorption method and used as detector probes. Using this approach, the signal response to the detection was further determined using a fluorescent signal intensity measurement method via a portable reader, in combination with fluorescence imaging analysis. The degree to which the fluorescence signal response is detected is proportional to the amount of the target virus protein present in the system, reflected by the accumulation of the formed particle–antibody conjugates within the test system. Under optimized conditions, the system is capable of detecting the influenza B virus protein at a level of 0.55 μg per test within 30 min, using small sample volumes as low as 100 μL (R² = 0.9544). In addition to its simplicity, further application of the system in detecting the influenza B virus protein was demonstrated using the viral transport media as specimen matrices. It was also shown that the system can perform the detection without cross-reactivity to other closely related respiratory viruses.