Rational Approach to Tailor Au–IrO₂ Nanoflowers as Colorimetric Labels for Lateral Flow Assays

Abstract

As the current pandemic has shown, lateral flow assays (LFAs) are a prime example of pointof-care devices enabling quick testing at an affordable price. However, their ease of use undeniably affects their sensitivity, making them less sensitive than other multi-step and time-consuming diagnostic assays, such as polymerase chain reactions and enzyme-linked immunosorbent assays. A possible solution to overcome this lack of sensitivity is the exploitation of bottom-up approaches to synthesize nanomaterials with outstanding properties for use as colorimetric labels in LFAs, that is, using nanoparticles with better optical capabilities to improve the generation of the colorimetric signal and the overall sensitivity of LFAs. Following this strategy, we rationally optimized the synthesis of gold and iridium oxide nanoflowers (Au−IrO 2 NFs) to enhance their physical−chemical properties as colorimetric labels in LFAs. Specifically, we were able to rationally control their size (from 155 to 53 nm in diameter) in order to guarantee an optimal flow along the different pads of a LFA. Moreover, thanks to their superior plasmonic behavior (compared to standard AuNPs), we could achieve an 8.5-fold lower limit of detection (down to 1.2 ng/mL) for human immunoglobulin G (HIgG) than standard LFAs (10.1 ng/mL). Therefore, due to their optical and redox properties, bioconjugation capabilities, and synergic combination of the individual components, Au−IrO 2 NFs appear as potential candidates for the next generation of optical LFAs.

Harnessing Bioluminescent Bacteria to Develop an Enzymatic-free Enzyme-linked immunosorbent assay for the Detection of Clinically Relevant Biomarkers

“…The collected data are in perfect agreement with previous studies. 47,53 Au−IrO 2 NFs have no toxic effect on bacteria and can enhance the IFE. We used turbidity measurements to investigate the toxicity of these nanoparticles by monitoring the growth of A. fischeri in the presence and in the absence of Au−IrO 2 NFs.…”

“…Of note, we selected the functionalization process and antibodies that have been characterized in our previous studies and successfully adapted to support bioanalytical platforms (i.e., LFAs ,, and electrochemical immunoanalysis). We used dynamic light scattering (DLS) and Z-potential to confirm that the AuNPs were functionalized and fully covered with the anti-HIgG after the conjugation process (Figure S5c and Table S1), as previously reported . Next, we characterized the analytical performance of the BBLISA (Figure ) by collecting the bioluminescent signal in the presence of increasing concentrations of the target (Figure a, orange curve; and Figure S6a).…”

“…For this purpose, we decided to use gold–iridium oxide nanoflowers (Au–IrO 2 NFs) for two reasons: they have a higher extinction coefficient and surface area than those of AuNPs, and we have recently optimized their synthesis and functionalization and fully characterized their morphology, composition, size, and stability . In addition, we demonstrate their ability to support target biorecognition in a paper-based sensing platform . Therefore, based on our previous experience, we synthesized Au–IrO 2 NFs, and we characterized them using TEM images (Figure b).…”

“…As expected, the nanoflowers exhibit a spherical morphology with a highly tortuous branched structure (Figure b) and an overall average diameter of 62.3 ± 5.3 nm resulting in a monodisperse size distribution (Figure S8a). Next, we demonstrate their superior absorption properties by comparing the UV–vis spectra of Au–IrO 2 NFs and AuNPs performed at the same nanoparticle concentrations (Figure a). Nanoflowers exhibit 25 times higher molar extinction coefficient than 20 nm of AuNPs, which is due to their surface flower-like branching morphology and their hybrid composition of gold and iridium .…”

“…Origin 2019-64 bit software was used for fitting curves using four parameter logistic equation. 42,46,47 The relative bioluminescence signal change (%) in BBLISA (Figures 2, 3 where X is the concentration of the target, BL max is the maximum value of the signal change (%) in the absence of target, BL min is the minimum value of the signal change (%) in the presence of the target, IC 50 is the concentration of target where is located the inflection point, and h is the Hill coefficient which describes the slope of the curve.…”