Localized Surface Plasmon Resonance (LSPR) is an optical biosensing technology used to detect and monitor molecular interactions at the surface of nanostructures. Because it enables real-time, label-free measurements and can function directly in complex sample matrices, LSPR has become a valuable tool for applications that require fast, reliable information without extensive sample preparation. At ArgusEye, LSPR forms the scientific basis of our Auga® platform for bioprocess monitoring. This article takes a closer look at the technology itself, how it works, what it can measure, and how it compares to other established analytical methods used in biopharmaceutical development and manufacturing.
What is LSPR used for?
Localized Surface Plasmon Resonance (LSPR) is used to detect and monitor molecular interactions at the surface of nanoparticles. In bioprocessing and life science applications, it can be used to measure changes in concentration, binding, or molecular interactions without relying on fluorescent or radioactive labels. A typical nanoplasmonic biosensor using localized surface plasmon resonance consists of metallic nanostructures whose optical response changes when molecules interact with their surface. By measuring this change, it is possible to follow events happening near the surface of the nanostructures. This is specifically useful when rapid, label-free detection is required, for example in bioprocesses. Depending on the sensor design, LSPR can support applications such as monitoring biomolecules or label-free detection of impurities in samples, in an at-line, on-line, or in-line setup.
One of the main advantages of a nanoplasmonic biosensor is that measurements can be relatively simple and fast, which is beneficial for applications where conventional laboratory analysis is too slow or requires extensive sample preparation. By utilizing the LSPR phenomenon in a sensor system, measurements can be brought closer to the process, allowing process information to be obtained without relying on offline laboratory analysis. This also means that nanoplasmonic biosensors are not limited to pure buffer conditions. The technology has been shown to function directly in complex matrices such as cell growth media, meaning the sensor can be placed directly on the process line without first purifying or diluting the sample to remove interference.
Label-free detection in bioprocesses is, however, only one application area for LSPR. In academic research, the same effect is also studied for entirely different purposes. The ability of nanostructures to absorb and concentrate light is used to drive photocatalytic reactions, generate localized heat for photothermal applications, and improve light absorption in solar cells. Sensing and non-sensing applications share the same physics but differ in purpose: sensing reads a signal, while non-sensing applications use the effect to perform work.
How does LSPR technology work?
Surface Plasmon Resonance (SPR) is a widely used technique for studying biomolecular interactions. It relies on surface plasmons, which are waves of electron oscillation that form at the interface between a metal and a surrounding material. When molecules bind to the surface, the local refractive index near the surface changes, causing a measurable shift in the plasmon signal.
Localized Surface Plasmon Resonance (LSPR) is based on the same principle but behaves differently at the nanoscale. Instead of the plasmon travelling along a metal surface, the electron oscillation is confined within a metallic nanostructure. When light interacts with the nanostructure, it excites these electrons and results in a distinct peak in the optical spectrum.
When molecules bind to the surface of the nanostructure, they change the local refractive index, causing the plasmon peak to shift. This shift can be measured and related to the amount of material binding to the surface. By attaching a specific ligand to the nanostructure, the sensor can be designed to detect a specific molecule.
The sensing range of LSPR is typically around 20 nm from the nanostructure surface, but varies depending on the nanostructure’s size, shape, and material. This small sensing volume makes LSPR particularly sensitive to changes occurring close to the surface, while reducing the influence of material further away from the nanostructures.
What does label-free detection mean, and why does it matter?
Label-free detection means measuring a molecule directly, without first attaching a detectable label to it, such as a fluorescent, radioactive, or enzyme-linked molecule. Label-free detection instead relies on the molecule’s own physical properties, which improves reliability since the measurement is based on direct binding between molecule and ligand. It also simplifies the assay by eliminating the need for labelling agents, reducing the risk of interference introduced by the labelling step itself. This reduces both time and reagent costs and allows binding events to be followed in real time, which is more difficult to achieve with methods requiring multiple addition and washing steps for label attachment. For bioprocess teams, this translates in practice to faster answers and fewer manual steps between sampling and outcome.
By comparison, a method such as ELISA requires a detection antibody, an enzyme substrate, and several incubation and washing steps before a result is obtained. A label-free method requires only one recognition element on the sensor surface, reducing sources of error and run-to-run variability, and shortening the path to a result.
How does LSPR compare with other bioprocess analytical methods?
Several analytical methods can be used to measure IgG titer in bioprocessing, but they differ significantly in how measurements are performed and how quickly results can be obtained. Established methods such as high-performance liquid chromatography (HPLC) and enzyme-linked immunosorbent assay (ELISA) provide reliable, well-characterized measurements, while label-free detection technologies such as biolayer interferometry (BLI) and LSPR offer the potential for faster analysis, enabling measurements to be performed closer to the process and reducing the time between sampling and results. The choice of analytical method is therefore linked to whether the priority is detailed offline characterisation or rapid, real-time process monitoring.
HPLC is currently considered the gold standard for titer measurement due to its high specificity and robustness. Another widely used method is ELISA. However, both HPLC and ELISA require substantial manual labour and sample preparation and have longer turnaround times. BLI, traditional SPR, and LSPR are label-free detection methods that offer the potential for more rapid and automated measurements. However, SPR has a relatively narrow and low dynamic range, which often requires samples to be heavily diluted before measurement. LSPR-based sensors, particularly in fibre-optic form, offer potential for real-time process monitoring while allowing integration directly in-line or on-line. This could reduce the need for at-line or off-line measurements and provide results closer to real time compared with classical analytical methods such as HPLC and ELISA.
Choosing the right analytical method often comes down to weighing priorities, how critical absolute accuracy and long-term robustness are for a given decision, against how much value there is in getting a result quickly and close to the actual sampling time. Chromatography-based methods such as HPLC offer a well-characterised reference result, particularly valuable for regulatory and validation purposes. Label-free techniques like BLI, SPR, and LSPR also offer accurate, robust measurements, with the added advantage of shorter turnaround and the possibility of measuring closer to real time. The right choice, in the end, depends on whether the priority is a long-established reference method or a faster answer that can be acted on sooner.
What other biomolecules and applications can LSPR be used for?
Measuring antibody titer is an established use of LSPR in bioprocessing today, but it is far from the only application. The same underlying principle, capturing a molecule on the sensor surface and measuring the resulting shift in signal, can be adapted to detect many other types of molecules relevant to a certain process, provided a suitable capture material can be developed for the sensor surface.
Beyond antibody titer, LSPR can also be adapted to measure entirely different types of molecules. One example is the monitoring of cytokine secretion, such as IL-6, in real time, demonstrated by, among others, C. Zhu et al. at Osaka University (1). Similar plasmonic sensors have been applied to virus quantification as well, as for example an LSPR-based assay using antigen-coated gold nanoparticles to detect SARS-CoV-2 nucleocapsid protein directly, with results visible within minutes, developed by K. Behrouzi and L. Lin at the University of California, Berkeley (2).
This flexibility has been demonstrated across many different types of molecules, simply by changing the capture molecule on the sensor surface. Reported applications cover a wide range of analyte types such as proteins, nucleic acids, small molecules, and viral particles, each requiring its own capture chemistry rather than a different sensor principle. LSPR-based platforms have also been shown to remain compatible with complex matrices such as cell culture media and serum, which illustrates that LSPR is not restricted to a narrow set of biomolecules. The main constraints lie less in the sensor technology itself and more in developing a suitable, specific capture molecule for a given target.
This principle also suggests that the potential of LSPR and nanoplasmonic biosensors extends beyond bioprocess applications. In principle, the same label-free detection approach could be adapted to other sensing areas, such as water analysis, environmental monitoring, or food safety, where specific target molecules or particles need to be detected directly in complex samples. The key requirement remains the same: a suitable capture molecule or surface chemistry must be available to selectively bind the target of interest. This makes LSPR a versatile platform for label-free detection across different application areas, rather than a technology limited to a specific type of bioprocess or biomolecule.
References
- Zhu C, Luo X, Espulgar WV, Koyama S, Kumanogoh A, Saito M, Takamatsu H, Tamiya E. Real-Time Monitoring and Detection of Single-Cell Level Cytokine Secretion Using LSPR Technology. Micromachines (Basel). 2020 Jan 19;11(1):107. doi: 10.3390/mi11010107. PMID: 31963848; PMCID: PMC7019717.
- Behrouzi K, Lin L. Gold nanoparticle based plasmonic sensing for the detection of SARS-CoV-2 nucleocapsid proteins. Biosens Bioelectron. 2022 Jan 1;195:113669. doi: 10.1016/j.bios.2021.113669. Epub 2021 Sep 28. PMID: 34607117; PMCID: PMC8479426.
