表面等离子体共振（SPR）

Surface plasmon resonance (SPR) is a label-free optical phenomenon that detects biomolecular interactions on a metal-coated sensor without requiring chemical labels. Light reflects off a prism-metal interface at a specific angle called the SPR angle. As molecules bind to the metal surface, the electronic properties change, shifting this angle. By measuring these angle changes in real-time, researchers can monitor binding events and calculate kinetic data.

An SPR sensor consists of a thin noble metal layer on a prism face. Light reflects off the prism-metal interface into a photodetector. The reflected light has high intensity except at the surface plasmon resonance angle, which depends on the metal's electronic properties. When proteins bind to the surface, these properties change, shifting the angle. Measuring relative angle changes reveals binding kinetics and interaction rates.

SPR offers label-free detection, eliminating time-consuming labeling steps and avoiding potential alterations to analyte functionality. Real-time monitoring of biomolecular interactions occurs without chemical modifications. This approach reduces experimental complexity and provides direct kinetic measurements. SPR is faster and less disruptive than ELISA, making it ideal for studying sensitive proteins and binding interactions.

Localized SPR (LSPR) uses metal nanoparticles as the sensor surface instead of a continuous metal layer. The electronic properties affecting the SPR angle are highly localized to each nanoparticle. This localization improves sensitivity and signal resolution compared to standard SPR. LSPR enables high spatial resolution measurements and is compatible with complementary techniques like fluorescence microscopy.

Running and sample buffers are prepared first. The cleaned sensor chip is loaded into the instrument's flow cell. A substrate coating is applied to capture the ligand. The ligand flows over the sensor in running buffer and binds to the substrate. Then the analyte in sample buffer flows through, selectively binding to the immobilized ligand. Reflectance changes are plotted and compared against controls to determine rate constants.

SPR imaging (SPRi) directs reflected light to a CCD detector, producing high-contrast, high-resolution images of the entire sensor surface. This technique enables protein identification by creating characteristic reflectivity profiles versus time and receptor concentration. SPRi can answer questions about molecular affinity, kinetics, specificity, and concentration across multiple binding sites simultaneously.

Quantum dots, nanoscale semiconductors, serve as SPR signal enhancement agents mixed with the analyte. This nano-SPRi method significantly improves sensitivity and limit of detection compared to standard SPRi and ELISA. The enhancement maintains the speed advantage of SPR while achieving detection capabilities comparable to enzyme assays and study enzyme activity and kinetics methods, making it valuable for low-abundance biomolecule analysis.