Surface Plasmon Resonance Application for Bacteria Pathogen Detection: A Minireview

Detection of pathogenic bacteria requires a fast and accurate process so technological developments related to the sensitivity and selectivity of a sensor are very concerned. Surface Plasmon Resonance (SPR) technology has great potential to detect pathogenic bacteria that are not only used for medical diagnostics, but food safety surveillance and environmental monitoring are also applications of SPR. The success of SPR has proven the advantages of real-time detection without the need for fluorescent markers or additional dyes. The development of sensor surface systems of SPR with nanomaterials is one of the discussions in this paper. The recent development of SPR in recent years is summarized by linking future prospective applications.


Introduction
The threat to public health associated with foodborne diseases caused by bacterial pathogens has led to an urgent need to develop technologies to detect these pathogens quickly and reliably [1].In addition, toxin-producing anthrax in human pathogens [2], campylobacteriosis [3], bacterial meningitis [4], and food poisoning that are characteristic of pathogens from different bacteria can cause serious effects on humans [5,6].The effects of pathogenic bacteria on human health can have a serious impact on human health.In addition to fostering immunity in the body as antibodies after exposure, it can also potentially be a toxic disease [7,8].According to the Centers for Disease Control and Prevention, every year millions of people in the US are affected by foodborne illnesses, resulting in thousands of hospitalizations and deaths [8].As time goes by, bacteria are getting smarter to free themselves from antibiotics, which leads to the use of antibiotics out of control.Inappropriate use of antibiotics can result in antibiotic resistance.Bacteria mutate and develop drug immunity [9].However, environmentally friendly applications such as bacteriophages that do not infect humans are also being considered [10].In addition, the production of large quantities of bacteriophage is easy and inexpensive.These advantages make the detection of pathogenic bacteria in biosensor development a new biorecognition tool [10,11].
Early detection of pathogenic microorganisms is concerned with rapid identification.This is indispensable in infectious disease control.Traditional methods take about 7 to 8 days to detect a standard bacterium [12].In addition, this method has limitations in detecting low concentrations in samples.To overcome these challenges, various detection methods have been developed, including the wellestablished pathogen detection polymerase chain reaction [13], Culture and Colony Counting [14], and Biosensor technology such as immunoassay, potentiometric [15], piezoelectric [16], optical [17], and surface plasmon resonance (SPR).SPR belongs to the category of optical sensors that are proven to detect small molecules in blood [18], utilization of nanoparticles as capturers and surface analysis of antibodies to proteins in SPR have been developed [19].
This review provides an application of surface plasmon resonance as a pathogenic bacteria detection.Bacterial detection with SPR utilizes immunoassay, bacteriophage, and nanoparticles to detect bacteria with various kinds of pathogenic bacteria detection with SPR that have been proposed by previous researchers.In addition, the latest developments regarding SPR are also presented with a future perspective.

General SPR Instrumentation
SPR belongs to the category of optical sensors that are integrated for detection with high sensitivity and accuracy.SPR has a surface for microorganism detection that is connected to several layers in the biosensor system [20].This layer incorporates recognition receptors that interact with the analyte in the sample.The thin film often used in this layer is gold because it has good conductivity and optical properties that depend on particle size and shape [21,22].Figure 1.shows the flow system on the SPR surface consisting of flow cells to confine the sample.The SPR phenomenon occurs due to electromagnetic waves at the surface Plasmon (SP) which propagate through the interface between the metal and the dielectric material.The Attenuated Total Reflectance (ATR) method proposed by Kretschmann is the basis of the SPR phenomenon itself [24].This method uses a laser beam passed through a prism with a high refractive index.The laser beam will be totally refracted at the surface of the prism and reflected back.On the surface of the prism, there is a metal layer with a refractive index lower than the refractive index of the prism.When the laser light is reflected back from the metal surface, a surface plasmon resonance will occur.This resonance causes the light absorption by the metal to increase.The light absorption by the metal can be measured using a detector.This change in light absorption can be used to detect the presence of biomolecules.The refractive index in contact with the thin film surface will be directly related to the wavelength of light absorbed by the SPR.Therefore, the refractive index change at the surface plasmon (SP) is due to the binding of the analyte in the dielectric to the antibody, as shown in Figure 2. Evanescent waves appear and propagate along the metal film.This is because the optical primes are bypassed by the light waves and reflected completely.when the constant of evanescent wave propagation is equal to the outer boundary of the metal film on the surface plasmon, the coupling between the light wave and the surface plasmon occurs so that this condition can be expressed by: Where kx is the light wave vector (constant propagation), ϴ denotes the angle of incidence and λ is the wavelength of light [26].In fact, the propagation constant is always lower than the outer boundary of the metal film, so it is necessary to configure the ATR method to obtain equality between the two [27].The value of kx is affected by ε2 which is the constant of the dielectric layer, so changes in refractive index greatly affect the sensitivity of SPR.

SPR for Bacteria Pathogen Detection
The overall performance of the sensor is determined by surface chemistry, while the sensitivity of the sensor is determined by instrumentation.This surface consists of bioreceptor or molecular recognition elements (MREs) immobilized on a non-fouling background.High specificity, stability, and high affinity are the properties of MREs.Proteins [28], antibodies [29,30], whole cells [31] dan aptamers [32] are a part of MRE.A frequent application technique to improve the sensitivity and specificity of molecular recognition elements in Surface Plasmon resonance for bacterial pathogen detection is antibodies in immunoassay techniques.Besides antibodies, bacteriophages also have the advantage of associating certain parasitic viruses to replicate only in their host bacteria [33,34].Apart from MREs, nanoparticles are also a frequently used technique in modified SPR applications on dielectrics or sensor chips [35].This review focuses on SPR application techniques for bacteria pathogen detection using immunoassays, bacteriophage, and nanoparticles.

SPR based on Immunoassay
Immunoassay cannot be separated from antibodies as the most commonly used biorecognition.Before testing, antibodies must be prepared by production and purification of the antibody itself.Analyte testing needs to pay attention to the mass and amount of the analyte.Some examples of SPR detection based on Immunoassay are presented in Table 1.

Antibodies.
Polyclonal antibody (PAbs) and Monoclonal antibody (MAbs) is a type of antibody that has a large function in various applications [36].PAbs are usually immunized in animals such as sheep, goats, and rabbits.PAbs are often used for immunosensor-based pathogen detection [37].Hybridoma technology is used to produce MAbs with murine host selection for immunization [38].

Type of Assay.
Direct, Compatitive, Sandwich, Inhibitors are a type of assay that has been used in SPR applications.The mass variation of chemical and biological analytes requires SPR sensors to use a type of assay to facilitate detection.The direct analyte assay showed reasonable detection limits of approximately >10,000 Da [39,40].The insufficient refractive index change is due to the direct binding of low molecular weight analytes of about <1000 Da on the sensor surface.Therefore, an improvement in the detection limit is necessary for these analytes, such as in the sandwich assay [29], competitive [41], and inhibitor [42].Figure 3 illustrates the model of three types of detection assay in SPR.(a) Sandwich assay precedes the primary MRE to be immobilized on the sensor surface.The analyte is captured by the MRE when the sample is flowed through.The response depends on the mass and amount of the bound analyte.Then a secondary antibody is used so that the sensor response is amplified the detection limit is increased, and verification of the analyte occurs.(b) Competitive assay, where the target analyte is mixed with a conjugate analyte.(c) Inhibitor assay, the MRE concentration incubates the target analyte, and then analyte-free MRE will be detected on the surface.

SPR based on bacteriophage
The unique structure of phage such as the bond between the tail and the bacterial host is one of the advantages promised by this method (Figure 4).In addition, phages can be found in their host organisms with a long shelf life and cheaper production compared to antibodies.Phage only replicates in living bacterial cells, so it can determine whether the bacterial cells are alive or dead [50].Therefore, the approach with this method is promising for the direct detection of live pathogens [51].The efficiency in detecting pathogenic bacteria in this method is due to the immobilization factor of bacteriophage, allowing for detection at low concentrations [54].Some techniques in bacteriophage immobilization on a surface include chemical interaction with low risk for phage detachment from the substrate [50], physical adsorption with biosensing performance affected by the adsorption process so that the detachment of the substrate surface [55], and so on.The development of biosensors with this technique results in phage masses sufficient for the detection of pathogenic bacteria [56].SPR has high compatibility and sensitivity in terms of optical diagnostics for pathogenic bacteria detection.Table 2 is an example of SPR for this method.

SPR based on Nanoparticle
The application of nanoparticles in SPR requires material modifications to the dielectric layer or sensor chip.Nanoparticles that are to be functionalized to interact with samples containing pathogenic bacteria must be modified to bind to the biorecognition of the target [62].When the sample is injected with the nanoparticle and there is interaction with the bacteria on the dielectric surface above the metal surface, there is a change in the SPR response as shown in Figure 5.These changes correlate with the bacteria in the sample.Nanoparticle-based SPR detection is presented in Table 3. Nanoparticles have a tremendous impact on the advancement of both sensors and antimicrobial activity.Silver nanoparticles (CPAgNp) from orange peel extract can provide a reducing, stabilizing, and masking role for antimicrobial activity against pathogens.[66].

Recent Development and Future Perspective
Surface Plasmon Resonance has undergone further evolution and development.Conventional SPR only has the approach of detecting molecular interactions or structural changes on a surface through a thin layer of gold film placed on a prism (BK7) with readings only looking at the shift of a signal [24].However, the SPR applications that have been developed to date make it possible to visualize the interaction distribution on the surface [68].In addition, modification of SPR sensors and changing surface conditions for bacterial pathogen applications have been widely developed.Even the ultrasensitive detection of bacteria [69], ultrafast [70], and free labeled are very favorable SPR detection results [71].
Real-time analyses can be performed using either qualitative or quantitative methods.Currently, changes in the prism can determine the magnitude of the refractive index which affects the reading of the SPR sensor.The long-range mode of Surface Plasmon resonance is utilized to increase the luminescence upconversion up to 100-fold higher than the luminescence up-conversion of Ag film.[72].As research and insights from researchers evolve, a smartphone-based Surface Plasmon Resonance has been proposed [73].With the use of commercial smartphones and the development of studies on color development that depend on changes in the spacing of AuNPs.There are many benefits to be gained if this study is developed further.The tremendous development of SPR has only taken approximately 15 years with the latest update being smartphone-based SPR.In this era of digitalization, it is possible to experiment with SPR to be integrated with the internet.Modernization of SPR in the form of system optimization for faster detection, more sensitivity, and multi-detection is highly desired by researchers.Real-time sensor readings and detection results by utilizing microcontrollers integrated with the Internet of Things (IoT) will produce Surface Plasmon Resonance that is easy to use, cheap, flexible, and portable.In addition, material modification of the sensor using nanoparticle materials that have good applications greatly supports this future perspective.This also considers the commercial SPR which is very expensive, so by utilizing the application of instrument and control science on the Surface Plasmon Resonance sensor, it produces a better SPR than before.In addition, modifying the material on the surface of the SPR with nanoparticles that have good antimicrobial pathogen activity, allows to increase the sensitivity and speed in identifying pathogenic bacteria.This is very beneficial for researchers or medical personnel in dealing with the spread of a disease.

Conclusion
Immunoassay-based SPR detection results almost get different LODs in each application.The significant effect of modified antibodies is to detect the same bacteria, namely E. Coli.Antibody modification on E. Coli using Rabbit anti-E.coli antisera resulted in a much smaller LOD compared to other applications.This shows that the antibody and assay type significantly affect the identification of the target.The bacteriophage method has a bond that is affected by the process of detaching the surface of the substrate so it takes time in the detection.Detection of E. coli with T4 bacteriophage and MRSA using BP14 takes less than 20 minutes with the advantage of LoD 10^3.So it promises the detection of pathogenic bacteria with fast SPR and high sensitivity.Furthermore, the application of SPR by utilizing nanoparticles on the surface had a remarkable effect.The detection of Salmonella Typhimurium and E. Coli o157:H7 with LoDs of 4.7 log cfu/ml and 10 6 cfu/ml, respectively, showed that these nanoparticles were effective in detecting pathogenic bacteria with a high degree of sensitivity.Magnetic iron oxide nanoparticle material with monoclonal antibody 1C8 states that the use of nanoparticles with appropriate reagents to detect bacteria will increase the sensitivity and selectivity of the SPR surface.Thus, a good identification of SPR sensors will be obtained.The MoS2 nanosheet yielded an LOD of 94, offering an extra sensitive method.So, the role of nanoparticles has great potential in improving the detection ability of pathogenic bacteria.The combination of nanoparticles with reagents or antibodies can improve the LOD number, making it more effective in detecting a target analyte.

Table 1 .
Detection of SPR based on immunoassay.

Table 2 .
Detection of SPR based on bacteriophage.

Table 3 .
Detection of SPR based on nanoparticle.