Abstract
An electrochemical capacitance immunosensor based on an interdigitated wave-shaped micro electrode array (IDWµE) for direct and label-free detection of C-reactive protein (CRP) was reported. A self-assembled monolayer (SAM) of dithiobis (succinimidyl propionate) (DTSP) was used to modify the electrode array for antibody immobilization. The SAM functionalized electrode array was characterized morphologically by atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy (EDX). The nature of gold-sulfur interactions on SAM-treated electrode array was probed by X-ray photoelectron spectroscopy (XPS). The covalent linking of anti-CRP-antibodies onto the SAM modified electrode array was characterized morphologically through AFM, and electrochemically through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The application of phosphate-buffered saline (PBS) and human serum (HS) samples containing different concentrations of CRP in the electrode array caused changes in the electrode interfacial capacitance upon CRP binding. CRP concentrations in PBS and HS were determined quantitatively by measuring the change in capacitance (ΔC) through EIS. The electrode immobilized with anti-CRP-antibodies showed an increase in ΔC with the addition of CRP concentrations over a range of 0.01–10,000 ng mL−1. The electrode showed detection limits of 0.025 ng mL−1 and 0.23 ng mL−1 (S/N = 3) in PBS and HS, respectively. The biosensor showed a good reproducibility (relative standard deviation (RSD), 1.70%), repeatability (RSD, 1.95%), and adequate selectivity in presence of interferents towards CRP detection. The sensor also exhibited a significant storage stability of 2 weeks at 4 °C in 1× PBS.
Highlights
C-reactive protein (CRP; 118 kDa) is a homopentameric classical acute phase inflammatory protein [1]
To confirm DTSP-self-assembled monolayer (SAM) functionalization on the electrode array surface, energy dispersive X-ray spectroscopy (EDX) measurements were first conducted to study the elemental composition of the microelectrode array before and after SAM
Where the elements Au, Si, and Ti were from the electrode array surface and additional elements S and C on electrode surfaces rose due to SAM functionalization on the electrode array
Summary
C-reactive protein (CRP; 118 kDa) is a homopentameric classical acute phase inflammatory protein [1]. Many studies have reported the role of CRP as a potential biomarker for coronary heart disease, and likely as a direct contributor in vascular inflammation [3]. The reference level of CRP in the blood of healthy individuals is below 3 μg mL−1 and changes between 0 and 1 μg mL−1 (low risk), 1–3 μg mL−1 (intermediate risk), and 3–10 μg mL−1 (high risk) are indicative of potential cardiovascular events [4]. Pepys and Hirschfield reported that minute changes between 0.1 and 10 μg mL−1 are strongly associated with predictions of future coronary events, while larger increases are related to chronic inflammatory diseases, such as arthritis [5]. Detection of very low levels of CRP is needed for early detection of cardiovascular risk and inflammatory events. Many efforts are still ongoing to improve the detection limit, dynamic range, reliability, cost, and measurement speed of such analytical methods
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