View source for Surface plasmon resonance

{{Short description|Physical phenomenon of electron resonance}}
[[File:Surface Plasmon Resonance (SPR).jpg|thumb|400px|Surface plasmon resonance (SPR)]]

'''Surface plasmon resonance''' ('''SPR''') is a phenomenon that occurs where electrons in a thin metal sheet become excited by light that is directed to the sheet with a particular angle of incidence, and then travel parallel to the sheet. Assuming a constant light source wavelength and that the metal sheet is thin, the angle of incidence that triggers SPR is related to the refractive index of the material and even a small change in the refractive index will cause SPR to not be observed. This makes SPR a possible technique for detecting particular substances ([[analyte]]s) and SPR biosensors have been developed to detect various important biomarkers.<ref>{{Citation |last1=Zhu |first1=Xiaoli |title=Chapter 10 - Spectrometry |date=2019-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780128150535000106 |work=Nano-Inspired Biosensors for Protein Assay with Clinical Applications |pages=253 |editor-last=Li |editor-first=Genxi |publisher=Elsevier |language=en |isbn=978-0-12-815053-5 |access-date=2023-01-17 |last2=Gao |first2=Tao}}</ref><ref>{{cite journal |last1=Marques Lameirinhas |first1=Ricardo A. |last2=N. Torres |first2=João Paulo |last3=Baptista |first3=António |last4=Marques Martins |first4=Maria João |title=A new method to analyse the role of surface plasmon polaritons on dielectric-metal interfaces |journal=IEEE Photonics Journal |date=2022 |volume=14 |issue=4 |pages=1–9 |doi=10.1109/JPHOT.2022.3181967|doi-access=free |bibcode=2022IPhoJ..1481967L }}</ref>

==Explanation==
The [[surface plasmon polariton]] is a non-radiative [[Surface wave#Electromagnetic waves|electromagnetic surface wave]] <!--The field energy present in the dielectric medium is evanescent. --> that propagates in a direction parallel to the negative permittivity/dielectric material interface. Since the wave is on the boundary of the conductor and the external medium (air, water or vacuum for example), these oscillations are very sensitive to any change of this boundary, such as the adsorption of molecules to the conducting surface.<ref>{{cite journal | vauthors = Zeng S, Baillargeat D, Ho HP, Yong KT | title = Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications | journal = Chemical Society Reviews | volume = 43 | issue = 10 | pages = 3426–3452 | date = May 2014 | pmid = 24549396 | doi = 10.1039/C3CS60479A | hdl = 10356/102043 | hdl-access = free }}</ref>

To describe the existence and properties of surface plasmon polaritons, one can choose from various models (quantum theory, [[Drude model]], etc.). The simplest way to approach the problem is to treat each material as a homogeneous continuum, described by a frequency-dependent [[relative static permittivity|relative permittivity]] between the external medium and the surface. This quantity, hereafter referred to as the materials' "[[dielectric function]]", is the [[complex permittivity]]. In order for the terms that describe the electronic [[surface plasmon]] to exist, the real part of the dielectric constant of the conductor must be negative and its magnitude must be greater than that of the dielectric. This condition is met in the infrared-visible wavelength region for air/metal and water/metal interfaces (where the real dielectric constant of a metal is negative and that of air or water is positive).

LSPRs ([[localized surface plasmon]] resonances) are collective electron charge oscillations in metallic nanoparticles that are excited by light. They exhibit enhanced near-field amplitude at the resonance wavelength. This field is highly localized at the nanoparticle and decays rapidly away from the nanoparticle/dielectric interface into the dielectric background, though far-field scattering by the particle is also enhanced by the resonance. Light intensity enhancement is a very important aspect of LSPRs and localization means the LSPR has very high spatial resolution (subwavelength), limited only by the size of nanoparticles. Because of the enhanced field amplitude, effects that depend on the amplitude such as magneto-optical effect are also enhanced by LSPRs.<ref>{{cite journal | vauthors = González-Díaz JB, García-Martín A, García-Martín JM, Cebollada A, Armelles G, Sepúlveda B, Alaverdyan Y, Käll M | display-authors = 6 | title = Plasmonic Au/Co/Au nanosandwiches with enhanced magneto-optical activity | journal = Small | volume = 4 | issue = 2 | pages = 202–205 | date = February 2008 | pmid = 18196506 | doi = 10.1002/smll.200700594 | hdl-access = free | hdl = 10261/17402 | s2cid = 206490102 }}</ref><ref>{{cite journal|title=Evidence of localized surface plasmon enhanced magneto-optical effect in nanodisk array |journal=Appl. Phys. Lett. |volume=96|date=2010|page= 081915|doi=10.1063/1.3334726|bibcode = 2010ApPhL..96h1915D|issue=8| vauthors = Du GX, Mori T, Suzuki M, Saito S, Fukuda H, Takahashi M |doi-access=free}}</ref>

==Implementations==
[[File:Otto-schema.png|thumb|Otto configuration]]
[[File:SPR-schema.png|thumb|Kretschmann configuration]]

In order to excite surface plasmon polaritons in a resonant manner, one can use electron bombardment or incident [[light beam]] (visible and infrared are typical). The incoming beam has to match its [[momentum]] to that of the plasmon.<ref>{{cite journal|title=Size dependence of Au NP-enhanced surface plasmon resonance based on differential phase measurement |journal=Sensors and Actuators B: Chemical |date=2013|doi=10.1016/j.snb.2012.09.073|volume=176|pages=1128–1133| vauthors = Zeng S, Yu X, Law WC, Zhang Y, Hu R, Dinh XQ, Ho HP, Yong KT |url=https://www.researchgate.net/publication/268225952 }}</ref> In the case of [[Polarization (waves)|p-polarized]] light  (polarization occurs parallel to the plane of incidence), this is possible by passing the light through a block of glass to increase the [[wavenumber]] (and the [[momentum]]), and achieve the resonance at a given wavelength and angle. [[Polarization (waves)|S-polarized]] light (polarization occurs perpendicular to the plane of incidence) cannot excite electronic surface plasmons.
Electronic and magnetic surface plasmons obey the following [[dispersion relation]]:

:<math> k(\omega) = \frac{\omega}{c} \sqrt{\frac{\varepsilon_1 \varepsilon_2 \mu_1 \mu_2}{\varepsilon_1 \mu_1 + \varepsilon_2 \mu_2}} </math>

where k(<math>\omega</math>) is the wave vector, <math>\varepsilon</math> is the relative permittivity, and <math>\mu</math> is the relative permeability of the material (1: the glass block, 2: the metal film), while <math>\omega</math> is angular frequency and <math>{c}</math> is the speed of light in vacuum.<ref>{{cite journal |last1=Marques Lameirinhas |first1=Ricardo A. |last2=N. Torres |first2=João Paulo |last3=Baptista |first3=António |last4=Marques Martins |first4=Maria João |title=A New Method to Determine the Response of Kretschmann's Structure-Based Biosensors |journal=IEEE Sensors Journal |date=2022 |volume=22 |issue=21 |pages=20421–20429 |doi=10.1109/JSEN.2022.3207896|bibcode=2022ISenJ..2220421M |s2cid=252548497 }}</ref>

Typical metals that support surface plasmons are silver and gold, but metals such as copper, titanium or chromium have also been used.

When using light to excite SP waves, there are two configurations which are well known. In the '''Otto configuration''', the light illuminates the wall of a glass block, typically a prism, and is [[total internal reflection|totally internally reflected]]. A thin metal film (for example gold) is positioned close enough to the prism wall so that an [[evanescent wave]] can interact with the plasma waves on the surface and hence excite the plasmons.<ref name="modern-plasmonics"/>

In the '''Kretschmann configuration''' (also known as '''Kretschmann–Raether configuration'''), the metal film is evaporated onto the glass block. The light again illuminates the glass block, and an evanescent wave penetrates through the metal film. The plasmons are excited at the outer side of the film. This configuration is used in most practical applications.<ref name="modern-plasmonics">{{cite book | veditors = Maradudin AA, Sambles JR, Barnes WL |year=2014 |title=Modern Plasmonics |location=Amsterdam |publisher=[[Elsevier]] |pages= 1–23 |isbn=9780444595263}}</ref>

===SPR emission===
When the surface plasmon wave interacts with a local particle or irregularity, such as a [[surface roughness|rough surface]], part of the energy can be re-emitted as light. This emitted light can be detected ''behind'' the metal film from various directions.

=== Analytical implementations ===
Surface plasmon resonance can be implemented in analytical instrumentation. SPR instruments consist of a light source, an input scheme, a prism with analyte interface, a detector, and computer.

=== Detectors ===
The detectors used in surface plasmon resonance convert the photons of light reflected off the metallic film into an electrical signal. A position sensing detector (PSD) or charged-coupled device (CCD) may be used to operate as detectors.<ref>Bakhtiar, Ray. "Surface plasmon resonance spectroscopy: a versatile technique in a biochemist’s toolbox." Journal of Chemical Education 90.2 (2013): 203-209.</ref>

==Applications==
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Surface plasmons have been used to enhance the surface sensitivity of several spectroscopic measurements including [[fluorescence]], [[Raman scattering]], and [[second-harmonic generation]]. In their simplest form, SPR reflectivity measurements can be used to detect molecular adsorption, such as polymers, DNA or proteins, etc. Technically, it is common to measure the angle of minimum reflection (angle of maximum absorption). This angle changes in the order of 0.1° during thin (about nm thickness) film adsorption. (See also the Examples.) In other cases the changes in the absorption wavelength is followed.<ref>{{cite journal|title=A localized surface plasmon resonance based immunosensor for the detection of casein in milk|journal=Sci. Technol. Adv. Mater.|volume=8|date=2007|pages= 331–338|doi=10.1016/j.stam.2006.12.010|bibcode = 2007STAdM...8..331M|issue=4| vauthors = Hiep HM, Endo T, Kerman K, Chikae M, Kim DK, Yamamura S, Takamura Y, Tamiya E | display-authors = 6 |doi-access=|s2cid=136613827 }}</ref> The mechanism of detection is based on the adsorbing molecules causing changes in the local index of refraction, changing the resonance conditions of the surface plasmon waves. The same principle is exploited in the recently developed competitive platform based on loss-less dielectric multilayers ([[Distributed Bragg reflector|DBR]]), supporting surface electromagnetic waves with sharper resonances ([[Bloch surface waves]]).<ref>{{cite journal|journal=Sensors and Actuators B: Chemical|volume=174|pages=292–298|year=2012|doi=10.1016/j.snb.2012.07.015|title=Direct comparison of the performance of Bloch surface wave and surface plasmon polariton sensors| vauthors = Sinibaldi A, Danz N, Descrovi E, Munzert P, Schulz U, Sonntag F, Dominici L, Michelotti F }}</ref>

If the surface is patterned with different biopolymers, using adequate optics and imaging sensors (i.e. a camera), the technique can be extended to [[surface plasmon resonance microscopy|surface plasmon resonance imaging]] (SPRI). This method provides a high contrast of the images based on the adsorbed amount of molecules, somewhat similar to [[Brewster's angle|Brewster angle]] microscopy (this latter is most commonly used together with a [[Langmuir–Blodgett trough]]).

For nanoparticles, localized surface plasmon oscillations can give rise to the intense colors of [[suspension (chemistry)|suspensions]] or [[sol (colloid)|sols]] containing the [[nanoparticle]]s. Nanoparticles or nanowires of noble metals exhibit strong [[absorption band]]s in the [[ultraviolet]]–[[visible light]] regime that are not present in the bulk metal. This extraordinary absorption increase has been exploited to increase light absorption in photovoltaic cells by depositing metal nanoparticles on the cell surface.<ref>{{cite journal|journal=Journal of Applied Physics |volume=101|date=2007|doi=10.1063/1.2734885|title=Surface plasmon enhanced silicon solar cells|issue=9|pages=093105–093105–8|bibcode = 2007JAP...101i3105P| vauthors = Pillai S, Catchpole KR, Trupke T, Green MA |hdl=1885/16942|hdl-access=free}}</ref> The energy (color) of this absorption differs when the light is polarized along or perpendicular to the nanowire.<ref>{{cite journal|title=Phenomenological studies of optical properties of Cu nanowires|journal=Sci. Technol. Adv. Mater.|volume=8|date=2007|pages=277–281 |doi=10.1016/j.stam.2007.02.001|bibcode = 2007STAdM...8..277L|issue=4| vauthors = Locharoenrat K, Sano H, Mizutani G |doi-access=free}}</ref> Shifts in this resonance due to changes in the local index of refraction upon adsorption to the nanoparticles can also be used to detect biopolymers such as DNA or proteins. 
Related complementary techniques include plasmon waveguide resonance, [[Quartz crystal microbalance|QCM]], [[extraordinary optical transmission]], and [[dual-polarization interferometry]].

=== SPR immunoassay ===
[[File:SPR analysis of purified G6 scFv.jpg|thumb|454x454px|SPR analysis of the purified G6 scFv. '''(A)''' Illustrative schematic representing of G6 scFv immobilization on an MUA‐modified Ausensor. '''(B)''' SPR sensogram of the G6 scFv immobilization on an MUA‐modified Au‐sensor slide. '''(C)''' Sensogram of Hsp70 in related peptide.<ref name="Vostakolaei 14711–14724">{{Cite journal |last1=Vostakolaei |first1=Mehdi Asghari |last2=Molavi |first2=Ommoleila |last3=Hejazi |first3=Mohammad Saeid |last4=Kordi |first4=Shirafkan |last5=Rahmati |first5=Saman |last6=Barzegari |first6=Abolfazl |last7=Abdolalizadeh |first7=Jalal |date=September 2019 |title=Isolation and characterization of a novel scFv antibody fragments specific for Hsp70 as a tumor biomarker |url=https://onlinelibrary.wiley.com/doi/10.1002/jcb.28732 |journal=Journal of Cellular Biochemistry |language=en |volume=120 |issue=9 |pages=14711–14724 |doi=10.1002/jcb.28732 |pmid=30998271 |s2cid=121351794 |issn=0730-2312}}</ref> '''scFv''': a single-chain variable fragment of an antibody|alt=scFv: a single-chain variable fragment of an antibody]]
The first SPR [[immunoassay]] was proposed in 1983 by Liedberg, Nylander, and Lundström, then of the [[Linköping Institute of Technology]] (Sweden).<ref>{{cite journal|doi=10.1016/0250-6874(83)85036-7|title=Surface plasmon resonance for gas detection and biosensing|date=1983| vauthors = Liedberg B, Nylander C, Lunström I |journal=Sensors and Actuators|volume=4|pages=299–304}}</ref>  They adsorbed
human [[Immunoglobulin G|IgG]] onto a 600-Ångström silver film, and used the assay to detect anti-human IgG in water solution.  Unlike many other immunoassays, such as [[ELISA]], an SPR immunoassay is ''label free'' in that a [[Fluorescent labelling|label molecule]] is not required for detection of the analyte.<ref>{{cite journal | vauthors = Rich RL, Myszka DG | title = Higher-throughput, label-free, real-time molecular interaction analysis | journal = Analytical Biochemistry | volume = 361 | issue = 1 | pages = 1–6 | date = February 2007 | pmid = 17145039 | doi = 10.1016/j.ab.2006.10.040 }}</ref><ref>{{Cite journal |last1=Kordi |first1=Shirafkan |last2=Rahmati-Yamchi |first2=Mohammad |last3=Asghari Vostakolaei |first3=Mehdi |last4=Barzegari |first4=Abolfazl |last5=Abdolalizadeh |first5=Jalal |date=2019-02-21 |title=Purification of a Novel Anti-VEGFR2 Single Chain Antibody Fragmentand Evaluation of Binding Affinity by Surface Plasmon Resonance |url=https://apb.tbzmed.ac.ir/Abstract/apb-22975 |journal=Advanced Pharmaceutical Bulletin |language=en |volume=9 |issue=1 |pages=64–69 |doi=10.15171/apb.2019.008 |issn=2228-5881 |pmc=6468230 |pmid=31011559}}</ref><ref name="Vostakolaei 14711–14724"/> Additionally, the measurements on SPR can be followed real-time allowing the monitoring of individual steps in sequential binding events particularly useful in the assessment of for instance sandwich complexes.

===Material characterization===
[[Multi-parametric surface plasmon resonance]], a special configuration of SPR, can be used to characterize layers and stacks of layers. Besides binding kinetics, MP-SPR can also provide information on structural changes in terms of layer true thickness and refractive index. MP-SPR has been applied successfully in measurements of lipid targeting and rupture,<ref>{{cite journal | vauthors = Granqvist N, Yliperttula M, Välimäki S, Pulkkinen P, Tenhu H, Viitala T | title = Control of the morphology of lipid layers by substrate surface chemistry | journal = Langmuir | volume = 30 | issue = 10 | pages = 2799–2809 | date = March 2014 | pmid = 24564782 | doi = 10.1021/la4046622 }}</ref> CVD-deposited single monolayer of graphene (3.7Å)<ref>{{cite journal| vauthors = Jussila H, Yang H, Granqvist N, Sun Z |title=Surface plasmon resonance for characterization of large-area atomic-layer graphene film|journal=Optica|date=5 February 2016|volume=3|issue=2|pages=151|doi=10.1364/OPTICA.3.000151|bibcode=2016Optic...3..151J|doi-access=free}}</ref> as well as micrometer thick polymers.<ref>{{cite journal | vauthors = Korhonen K, Granqvist N, Ketolainen J, Laitinen R | title = Monitoring of drug release kinetics from thin polymer films by multi-parametric surface plasmon resonance | journal = International Journal of Pharmaceutics | volume = 494 | issue = 1 | pages = 531–536 | date = October 2015 | pmid = 26319634 | doi = 10.1016/j.ijpharm.2015.08.071 }}</ref>

===Data interpretation===
The most common data interpretation is based on the [[Fresnel equations|Fresnel formulas]], which treat the formed thin films as infinite, continuous dielectric layers. This interpretation may result in multiple possible [[refractive index]] and thickness values. Usually only one solution is within the reasonable data range. In [[multi-parametric surface plasmon resonance]], two SPR curves are acquired by scanning a range of angles at two different wavelengths, which results in a unique solution for both thickness and refractive index.

Metal particle plasmons are usually modeled using the [[Mie scattering]] theory.

In many cases no detailed models are applied, but the sensors are calibrated for the specific application, and used with [[interpolation]] within the calibration curve.

=== Novel applications ===
Due to the versatility of SPR instrumentation, this technique pairs well with other approaches, leading to novel applications in various fields, such as biomedical and environmental studies.

When coupled with [[nanotechnology]], SPR biosensors can use nanoparticles as carriers for therapeutic implants. For instance, in the treatment of [[Alzheimer's disease]], [[nanoparticle]]s can be used to deliver therapeutic molecules in targeted ways.<ref>{{cite journal | vauthors = Canovi M, Lucchetti J, Stravalaci M, Re F, Moscatelli D, Bigini P, Salmona M, Gobbi M | display-authors = 6 | title = Applications of surface plasmon resonance (SPR) for the characterization of nanoparticles developed for biomedical purposes | journal = Sensors | volume = 12 | issue = 12 | pages = 16420–16432 | date = November 2012 | pmid = 23443386 | pmc = 3571790 | doi = 10.3390/s121216420 | bibcode = 2012Senso..1216420C | doi-access = free }}</ref> In general, SPR biosensing is demonstrating advantages over other approaches in the biomedical field due to this technique being label-free, lower in costs, applicable in point-of-care settings, and capable of producing faster results for smaller research cohorts.

In the study of environmental pollutants, SPR instrumentation can be used as a replacement for former chromatography-based techniques. Current pollution research relies on [[chromatography]] to monitor increases in pollution in an ecosystem over time. When SPR instrumentation with a Kretschmann prism configuration was used in the detection of chlorophene, an emerging pollutant, it was demonstrated that SPR has similar precision and accuracy levels as chromatography techniques.<ref>{{cite journal | vauthors = Quintanilla-Villanueva GE, Luna-Moreno D, Blanco-Gámez EA, Rodríguez-Delgado JM, Villarreal-Chiu JF, Rodríguez-Delgado MM | title = A Novel Enzyme-Based SPR Strategy for Detection of the Antimicrobial Agent Chlorophene | journal = Biosensors | volume = 11 | issue = 2 | pages = 43 | date = February 2021 | pmid = 33572259 | pmc = 7915018 | doi = 10.3390/bios11020043 | doi-access = free }}</ref> Furthermore, SPR sensing surpasses chromatography techniques through its high-speed, straightforward analysis.

==Examples==

===Layer-by-layer self-assembly===
[[File:SPR-adsorption-data.png|thumb|SPR curves measured during the adsorption of a [[polyelectrolyte]] and then a [[clay]] mineral [[self-assembly|self-assembled]] film onto a thin (ca. 38 nanometers) gold sensor.]]

One of the first common applications of surface plasmon resonance spectroscopy was the measurement of the thickness (and refractive index) of adsorbed self-assembled nanofilms on gold substrates. The resonance curves shift to higher angles as the thickness of the adsorbed film increases. This example is a 'static SPR' measurement.

When higher speed observation is desired, one can select an angle right below
the resonance point (the angle of minimum reflectance), and measure the reflectivity changes at that point.
This is the so-called 'dynamic SPR' measurement. The interpretation of the data assumes that the structure of the film does not change significantly during the measurement.

===Binding constant determination===
[[File:SPR-curve.png|thumb|Association and dissociation signal]]
[[File:Biacore diagram.jpg|thumb|Example of output from [[Biacore]]]]
SPR can be used to study the real-time kinetics of molecular interactions. Determining the affinity between two [[ligand]]s involves establishing the [[Dissociation constant|equilibrium dissociation]] constant, representing the equilibrium value for the product quotient. This constant can be determined using dynamic SPR parameters, calculated as the dissociation rate divided by the association rate.

<math>K_{\rm D} = \frac{k_{\text{d}}}{k_{\text{a}}}</math>

In this process, a ligand is immobilized on the [[dextran]] surface of the SPR crystal. Through a [[microfluidics|microflow]] system, a solution with the analyte is injected over the ligand-covered surface. The binding of the analyte to the ligand causes an increase in the SPR signal (expressed in response units, RU). Following the association time, a solution without the analyte (typically a buffer) is introduced into the microfluidics to initiate the dissociation of the bound complex between the ligand and analyte. As the analyte dissociates from the ligand, the SPR signal decreases. From these association ('on rate', {{math|''k''<sub>a</sub>}}) and dissociation rates ('off rate', {{math|''k''<sub>d</sub>}}), the equilibrium dissociation constant ('binding constant', {{math|''K''<sub>D</sub>}}) can be calculated.

The detected SPR signal is a consequence of the electromagnetic 'coupling' of the incident light with the surface plasmon of the gold layer. This interaction is particularly sensitive to the characteristics of the layer at the gold–solution interface, which is usually just a few nanometers thick. When substances bind to the surface, it alters the way light is reflected, causing a change in the reflection angle, which can be measured as a signal in SPR experiments. One common application is measuring the kinetics of [[Antigen-antibody interaction|antibody-antigen interactions]].

===Thermodynamic analysis===
As SPR biosensors facilitate measurements at different temperatures, thermodynamic analysis can be performed
to obtain a better understanding of the studied interaction. By performing measurements at different temperatures,
typically between 4 and 40&nbsp;°C, it is possible to relate association and dissociation rate constants with activation
energy and thereby obtain thermodynamic parameters including binding enthalpy, binding entropy,
Gibbs free energy and heat capacity.

===Pair-wise epitope mapping===
As SPR allows real-time monitoring, individual steps in sequential binding events can be thoroughly assessed when investigating the suitability between antibodies in a sandwich configuration. Additionally, it allows the mapping of epitopes as antibodies of overlapping epitopes will be associated with an attenuated signal compared to those capable of interacting simultaneously.

== Innovations ==

=== Magnetic plasmon resonance ===
Recently, there has been an interest in magnetic surface plasmons. These require materials with large negative magnetic permeability, a property that has only recently been made available with the construction of [[metamaterial]]s.

=== Graphene ===
Layering [[graphene]] on top of gold has been shown to improve SPR sensor performance.<ref>{{Cite journal |last1=Nurrohman |first1=Devi Taufiq |last2=Chiu |first2=Nan-Fu |date=2021-01-15 |title=A Review of Graphene-Based Surface Plasmon Resonance and Surface-Enhanced Raman Scattering Biosensors: Current Status and Future Prospects |journal=Nanomaterials |volume=11 |issue=1 |pages=216 |doi=10.3390/nano11010216 |issn=2079-4991 |pmc=7830205 |pmid=33467669 |doi-access=free }}</ref> Its high [[Electrical resistivity and conductivity|electrical conductivity]] increases the sensitivity of detection. The large surface area of graphene also facilitates the immobilization of biomolecules while its low refractive index minimizes its interference. Enhancing SPR sensitivity by incorporating graphene with other materials expands the potential of SPR sensors, making them practical in a broader range of applications. For instance, the enhanced sensitivity of graphene can be used in conjunction with a silver SPR sensor, providing a cost-effective alternative for measuring glucose levels in urine.<ref>{{Cite journal |last1=Yadav |first1=Archana |last2=Mishra |first2=Madhusudan |last3=Tripathy |first3=Sukanta K. |last4=Kumar |first4=Anil |last5=Singh |first5=O. P. |last6=Sharan |first6=Preeta |date=2023-12-01 |title=Improved Surface Plasmon Effect in Ag-based SPR Biosensor with Graphene and WS2: An Approach Towards Low Cost Urine-Glucose Detection |url=https://doi.org/10.1007/s11468-023-01945-3 |journal=Plasmonics |language=en |volume=18 |issue=6 |pages=2273–2283 |doi=10.1007/s11468-023-01945-3 |s2cid=259932223 |issn=1557-1963}}</ref>

Graphene has also been shown to improve the resistance of SPR sensors to high-temperature annealing up to 500&nbsp;°C.<ref>{{Cite journal |last1=Jungnickel |first1=Robert |last2=Mirabella |first2=Francesca |last3=Stockmann |first3=Jörg Manfred |last4=Radnik |first4=Jörg |last5=Balasubramanian |first5=Kannan |date=January 2023 |title=Graphene-on-gold surface plasmon resonance sensors resilient to high-temperature annealing |journal=Analytical and Bioanalytical Chemistry |language=en |volume=415 |issue=3 |pages=371–377 |doi=10.1007/s00216-022-04450-4 |issn=1618-2642 |pmc=9829571 |pmid=36447098}}</ref>

=== Fiber-optic SPR ===
Recent advancements in SPR technology have given rise to novel formats increasing the scope and applicability of SPR sensing. [[Optical fiber|Fiber optic]] SPR involves the integration of SPR sensors onto the ends of optical fibers, enabling the direct coupling of light with the surface plasmons as the analytes are passed through a hollow SPR core.<ref>{{Cite journal |title=Optimum Design of Surface Plasmon Resonance (SPR) Tapered Fiber Optic Biosensing Probe With Graphene–MoS2 Over Layers for DNA Hybridization  |date=2022 |url=https://ieeexplore.ieee.org/document/9923639 |access-date=2023-11-25 |doi=10.1109/TPS.2022.3211645 |last1=Raghuwanshi |first1=Sanjeev Kumar |last2=Pandey |first2=Purnendu Shekhar |journal=IEEE Transactions on Plasma Science |volume=50 |issue=11 |pages=4767–4774 |bibcode=2022ITPS...50.4767R |s2cid=253318606 }}</ref> This format offers enhanced sensitivity and allows for the development of compact sensing devices, making it particularly valuable for applications requiring remote sensing in the field.<ref>{{Cite journal |title=High Sensitivity Surface Plasmon Resonance Magnetic Field Sensor Based on Au/Gold Nanoparticles/Magnetic Fluid in the Hollow Core Fiber  |date=2023 |url=https://ieeexplore.ieee.org/document/10123389 |access-date=2023-11-25 |doi=10.1109/JSEN.2023.3273708 |last1=Zhang |first1=Qi |last2=Liu |first2=Hailian |last3=Fu |first3=Rao |last4=Li |first4=Bin |last5=Yan |first5=Xin |last6=Zhang |first6=Xuenan |last7=Wang |first7=Fang |last8=Cheng |first8=Tonglei |journal=IEEE Sensors Journal |volume=23 |issue=12 |pages=12899–12905 |bibcode=2023ISenJ..2312899Z |s2cid=258648690 }}</ref> It also offers an increased [[surface area]] for analytes to bind to the inner lining of the [[Optical fiber|fiber optic]].

== See also ==
*[[Hydrogen sensor]]
*[[Multi-parametric surface plasmon resonance]]
*[[Nano-optics]]
*[[Plasmon]]
*[[Spinplasmonics]]
*[[Surface plasmon polariton]]
*[[Waves in plasmas]]
*[[Localized surface plasmon]]
*[[Quartz crystal microbalance]]

== References ==
{{reflist}}

== Further reading ==
{{refbegin}}
*[http://www.iop.org/EJ/abstract/1367-2630/10/10/105001 A selection of free-download papers on Plasmonics in New Journal of Physics]
*{{Cite journal| vauthors = Raether H |title= Surface plasmons on smooth and rough surfaces and on gratings|journal= Springer Tracts in Modern Physics|volume= 111|date=1988|isbn=978-3-540-17363-2 |bibcode= 1988STMP..111.....R|doi= 10.1007/BFb0048317|author-link= Heinz Raether}}
* {{cite book| vauthors = Maier S |title=Plasmonics: Fundamentals and Applications|publisher=Springer|date=2007|isbn=978-0-387-33150-8}}
* {{cite book| veditors = Schasfoort RB, Tudos AJ | title= Handbook of Surface Plasmon Resonance| publisher=RSC publishing|date=2008|isbn=978-0-85404-267-8 }}

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