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Cardiovascular diseases (CVDs) refer to diseases that affect the heart and blood vessels. CVDs are considered silent killers, which are fatal (death occurs abruptly) particularly when their diagnosis is delayed. Among the biomarkers related to cardiovascular diseases, C-reactive protein (CRP) is expressed or found in high concentrations during a cardiac event. Therefore, CRP is an excellent biomarker to diagnose cardiac events, and therefore quantitative monitoring of CRP is necessary. Herein, we report the fabrication of an immunosensor for the detection and monitoring of CRP. The anti-CRP monoclonal antibody (anti-CRP-mAb), which is specific to the CRP antigen, was immobilized on the surface of gold pre-modified with 4-mercaptophenyl boronic acid (MPBA) to act as a capture antibody. The self-assembled monolayer (SAM) of 4-mercaptophenyl boronic acid (4-MPBA), Au-MPBA, was used for the oriented immobilization of anti-CRP-mAb for piezoelectric (mass-sensitive) and piezoelectric (impedance) measurements. Controlling the orientation of anti-CRP-mAb was crucial to eliminate false positive and negative results during sample analysis. The quartz-crystal microbalance with dissipation (QCM-D) measurements enabled us to follow the covalent immobilization of anti-CRP-mAb on AuCQC-MPBA in real-time. QCM-D was further used to follow the affinity reactions between anti-CRP-mAb and CRP and further with the anti-CRP polyclonal antibody (anti-CRP-pAb). The changes in frequency (Δf, Hz) were related to the changes in the mass (Δm, ng cm−2) of CRP up to a concentration of 0.10 μg mL−1, which is equivalent to 0.10 mg L−1. CRP was detected using the direct affinity immunoassay (anti-CRP-mAb < CRP) and also a sandwich immunoassay using anti-CRP-pAb for signal enhancement. The developed piezoelectric CRP detection protocol was translated to a gold disc electrode for electrochemical impedance spectroscopy (EIS) measurements. The limit of detection (LOD) using both methods was at the μg mL−1or mg L−1level. Furthermore, the developed electrode could be regenerated using acidic buffer (0.10 M HCl). The detected signal could be reproduced to within 5% relative standard deviation (% RSD) in buffer and serum samples, showing excellent selectivity and specificity toward CRP.

Nanomagnet-silica shell (Fe3O4@SiO2) decorated with Au@Pd nanoparticles (NPs) were synthesized successfully. The characterization of Fe3O4@SiO2-NH2-Au@PdNPs was achieved using several spectroscopic and microscopic techniques. The quantitative surface analysis was confirmed using X-ray photoelectron spectroscopy. The Fe3O4@SiO2-NH2-Au@Pd0.30NPs exhibited excellent peroxidase-like activity by effectively catalyzing the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H2O2. The absorption peaks at 370 and 652 nm confirmed the peroxidase-like activity of the Fe3O4@SiO2-NH2-Au@Pd0.30NPs. The Michaelis–Menten constant (Km) of 0.350 and 0.090 mM showed strong affinity toward H2O2and TMB at Fe3O4@SiO2-NH2-Au@Pd0.30NPs. The mechanism of the peroxidase-like activity was found to proceed via an electron transfer process. A simple colorimetric sensor based on glucose oxidase and Fe3O4@SiO2-NH2-Au@Pd0.30NPs showed excellent selectivity and sensitivity towards the detection of glucose. The fabricated glucose biosensor exhibited a wide linear response toward glucose from 0.010 to 60.0 μM with an limit of detection of 60.0 nM and limit of quantification of 200 nM. The colorimetric biosensor based on Fe3O4@SiO2-NH2-Au@Pd0.30NPs as a peroxidase mimic was also successfully applied for the determination of glucose concentrations in serum samples. The synthesized Fe3O4@SiO2-NH2-Au@Pd0.30NPs nanozymes exhibited excellent potential as an alternative to horseradish peroxidase for low-cost glucose monitoring.