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A crucial step in the development of implanted medical devices, in vivo diagnostics, and microarrays is the effective prevention of nonspecific protein adsorption from real-world complex media such as blood plasma or serum. In this work, a zwitterionic poly(carboxybetaine acrylamide) (polyCBAA) biomimetic material was employed to create a unique biorecognition coating with an ultralow fouling background, enabling the sensitive and specific detection of proteins in blood plasma. Conditions for surface activation, protein immobilization, and surface deactivation of the carboxylate groups in the polyCBAA coating were determined. An antibody-functionalized polyCBAA surface platform was used to detect a target protein in blood plasma using a sensitive surface plasmon resonance (SPR) sensor. A selective protein was directly detected from 100% human blood plasma with extraordinary specificity and sensitivity. The total nonspecific protein adsorption on the functionalized polyCBAA surface was very low (<3 ng/cm (2) for undiluted blood plasma). Because of the significant reduction of nonspecific protein adsorption, it was possible to monitor the kinetics of antigen-antibody interactions in undiluted blood plasma. The functionalization effectiveness and detection characteristics using a cancer protein marker candidate of polyCBAA were compared with those of the conventional nonfouling oligo(ethylene glycol)-based surface chemistry.

Detection of single analyte molecules without the use of any label would improve the sensitivity of current biosensors by orders of magnitude to the ultimate graininess of biological matter. Over two decades, scientists have succeeded in pushing the limits of optical detection to single molecules using fluorescence. However, restrictions in photophysics and labelling protocols make this technique less attractive for biosensing. Recently, mechanisms based on vibrational spectroscopy, photothermal detection, plasmonics and microcavities have been explored for fluorescence-free detection of single biomolecules. Here, we show that interferometric detection of scattering (iSCAT) can achieve this goal in a direct and label-free fashion. In particular, we demonstrate detection of cancer marker proteins in buffer solution and in the presence of other abundant proteins. Furthermore, we present super-resolution imaging of protein binding with nanometer localization precision. The ease of iSCAT instrumentation promises a breakthrough for label-free studies of interactions involving proteins and other small biomolecules. T here are more than 2,000 proteins secreted by the human body at concentrations that are below the detection limit of the current commercial biosensors 1 . To achieve a sufficient sensitivity for early and robust diagnosis of health hazards such as cancer and lethal infectious diseases, scientists have explored a fury of strategies based on mechanical 2 , electrochemical 3 and optical 4,5 interactions. The ultimate performance would be to count the analyte molecules one at a time. Furthermore, a practical biosensor would ideally detect the molecule of interest directly and without the need for labelling, amplification or sophisticated architectures.A well-established approach for label-free detection is via the vibrational signatures of molecules, for example via Raman spectroscopy. This method has an exquisite selectivity, but singlemolecule sensitivity has only been reached by enhancing the Raman cross-section in the near field of surfaces 6 or tips 7 . A common alternative strategy relies on the detection of the refractive index change due to analyte binding. The oldest and commercially available implementation of this technique records the shift of the plasmon resonance of a gold-coated prism 8 , whereby the specificity and selectivity are provided by surface chemistry 9 . While submonolayer detection is readily achievable in such a device, a single molecule does not manifest a significant change in the refractive index of the sensor medium. To increase the sensitivity, confined modes of plasmonic nanostructures [10][11][12] and dielectric microresonators 13,14 have been investigated, but these techniques are intrinsically limited. First, there is a compromise between the size of the sensor active area and its sensitivity. Higher sensitivity comes through confined intensity distributions and, thus, at the cost of more restricted active area, fewer binding receptors and thinner functionalization 15 . Se...

We report on a unified theoretical model of the resolution of SPR sensors which makes it possible to predict the ultimate performance of all major configurations of SPR sensors. The theory indicates that the performance of SPR sensors is independent of the method of excitation of surface plasmons (prism or grating coupling) or the method of modulation (amplitude, angular or wavelength) and depends dominantly on the noise properties of the light source and detector. Results of the theoretical analysis are compared with the performance reported for several SPR sensors to illustrate that the best state-of-art SPR sensors are approaching their theoretical limits. Possibilities for further advances in the performance of SPR sensor technology are discussed.

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Surface plasmon resonance (SPR) biosensors belong to label-free optical biosensing technologies. The SPR method is based on optical measurement of refractive index changes associated with the binding of analyte molecules in a sample to biorecognize molecules immobilized on the SPR sensor. Since late 1990's, SPR biosensors have become the main tool for the study of biomolecular interactions both in life science and pharmaceutical research. In addition, they have been increasingly applied in the detection of chemical and biological substances in important areas such as medical diagnostics, environmental monitoring, food safety and security. This chapter reviews the main principles of SPR biosensor technology and discusses applications of this technology for rapid, sensitive and specific detection of chemical and biological analytes.

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