Due to rapidly rising rates of diabetes and prediabetic conditions worldwide and the associated lethal complications, it is imperative to devise new diagnostic tools that reliably and directly measure insulin levels in clinical samples. Herein, we report a simple and sensitive direct imaging of insulin levels in diabetic patient samples using a surface plasmon resonance microarray imager (SPRi). To enhance sensitivity, we utilized magnetic nanoparticles (MNPs) to capture insulin from serum samples either directly or via a capture antibody immobilized on MNPs. The insulin-captured nanoparticles were allowed to bind surface insulin-antibody for detection from pixel intensity increase using a charge coupled device (CCD) built-in with the SPRi. We have compared the analytical figures-of-merit of the SPRi immunoarray on detecting insulin prepared in various percentages of serum solutions. A four parameter logistic model was used to obtain the best fit of microarray responses with insulin concentration and indicated the cooperative binding of insulin–nanoparticle conjugates to surface antibody in both the buffer insulin and the serum insulin conjugates with MNPs. The cooperativity effect is attributed to the greater association of magnetic nanoparticle-bound insulin molecules with increasing concentration of insulin binding to surface antibody. This is the first report of an SPRi immunoarray to accomplish clinical diagnosis of diabetic and prediabetic conditions based on insulin levels with serum matrix effect analysis and comparison between direct and sandwich insulin assay formats.

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Surface plasmon resonance (SPR) spectroscopy is a label-free method for monitoring binding events among biomolecules in real time and for sensing applications.1 SPR spectroscopy has been widely used in chemistry, biochemistry, and biology research. Our objective is to achieve ultra-low level detection of small molecules that are important markers of cancer disease. Also, we are interested in monitoring pH changes in less abundant biological samples. As we know, pH is an essential parameter in biological, medical and industrial applications. Monitoring the pH level in biological fluids reflects the abnormal conditions in human body.2 , 3We will present our findings on sensitive detection of small molecules and amplification of SPR signals by using chemical indicators and detection by an SPR-array imager Acknowledgements.This project was supported by Oklahoma State University. 1.         Fasoli, J. B.; Corn, R. M., Surface Enzyme Chemistries for Ultrasensitive Microarray Biosensing with SPR Imaging. Langmuir 2015, 10.1021/la504797z. 2.         Sim, J.; Kwon, D.-S.; Kim, J., Acid-sensitive pH sensor using electrolysis and a microfluidic channel for read-out amplification. RSC Advances 2014, 4 (75), 39634-39638. 3.         Kick, A.; Mertig, M., Characterization of pH-sensitive polymer layers by surface plasmon resonance and quartz crystal microbalance. physica status solidi (a), 2015, 10.1002/pssa.201431604.