A quartz crystal microbalance (QCM) biosensor based on polydopamine (PDA) surface was developed for real-time analysis of the binding kinetics of protein-protein interactions. The biosensor was fabricated by simply immersing the gold sensor chip into an aqueous dopamine solution at pH 8.5 leading to a spontaneous deposition of PDA film onto the sensor chip surface, which was followed by incubation with the protein to immobilize it onto the PDA-coated sensor chip surface via Michael addition and/or Schiff base reactions. In this paper, the interaction between monoclonal anti-myoglobin 7005 antibody (IgG1) and its antigen human cardiac myoglobin was used as a model system for real-time analysis of biomolecule interactions on the biosensor surface. The kinetic parameters of the interaction between anti-myoglobin 7005 and myoglobin were studied on the biosensor surface, which were consistent with the results obtained via amine coupling. The biosensor based on PDA surface has excellent regenerability, reproducibility, and specificity. Compared with the most frequently/typically used amine coupling method for immobilization of proteins on carboxylated substrates, the modification methodology presented in this paper is simple, mild and is not subjected to the limitations of the isoelectric point (pI) of the protein. In addition, the PDA biosensor chip can be easily reused, which makes QCM biosensor analysis more efficient and cost effective.

Quartz crystal microbalance (QCM) has been extensively applied in real-time and label-free biomolecular interaction studies. However, the sensitive detection by QCM technology remains challenging, mainly due to the limited surface immobilization capacity. Here, a three-dimensional (3D) carboxymethyl dextran coated gold sensor chip surface was successfully fabricated with dextran of different molecular weight (100, 500 and 2000 kDa, respectively). To evaluate the 3D carboxymethyl dextran surface immobilization capacity, the 3D surface was used for studying antigen–antibody interactions on the QCM biosensor. The results showed that the protein immobilization capacity of the 3D carboxymethyl dextran (2000 kDa) surface exceeded more than 4 times the capacity of the 2D carboxyl surface, and 2 times the capacity of the traditional 3D carboxymethyl dextran (500 kDa) surface. Furthermore, the kinetic and affinity properties of antigen–antibody interactions were performed. Most notably, the optimized 3D carboxymethyl dextran (2000 kDa) surface could be used for small molecule detection, where the binding of biotinylated oligo (0.67 kDa) reached 8.1 Hz. The results confirmed that a 3D carboxymethyl dextran (2000 kDa) surface can be exploited for sensitive detection of low molecular weight analytes, which have great potential applications for characterizing the interactions between small molecule drugs and proteins.

TIMP-1 (Tissue Inhibitor of Metalloproteinases-1) is a protein involved in regulating extracellular matrix (ECM) degradation. It is recognized as a significant biomarker for cancer diagnosis. This study aimed to develop and characterize a single-stranded DNA (ssDNA) aptamer targeting human TIMP-1 protein with high affinity and specificity. A magnetic beads-based SELEX process combined with qPCR was used to select aptamers over seven rounds. The enriched ssDNA library was analyzed using high-throughput sequencing to identify candidate sequences, and these sequences were characterized using surface plasmon resonance (SPR) and binding assays to evaluate their affinity and specificity. The selected ssDNA aptamer demonstrated a dissociation equilibrium constant (K _D) of 0.41 nM and a very slow off-rate, enabling effective capture of TIMP-1 in serum samples. Furthermore, a chemiluminescent aptasensor was developed for TIMP-1 detection, which exhibited high specificity and a broad linear detection range from 1 to 500 ng/mL in human serum. The developed ssDNA aptamer targeting TIMP-1 shows high affinity and specificity, and the chemiluminescent aptasensor demonstrates promising potential for clinical diagnosis of TIMP-1 levels in human serum. Graphical abstract