Localized Surface Plasmon Resonance Sensing and its Interplay with Fluidics

In this Feature Article, we discuss the interplay between fluidics and the localized surface plasmon resonance (LSPR) sensing technique, primarily focusing on its applications in the realm of bio/chemical sensing within fluidic environments. Commencing with a foundational overview of LSPR principles from a sensing perspective, we subsequently showcase the development of a streamlined LSPR chip integrated with microfluidic structures. This integration opens the doors to advanced experiments involving fluid dynamics, greatly expanding the scope of LSPR-based research. Our discussions then turn to the practical implementation of LSPR and microfluidics in real-time biosensing, with a specific emphasis on monitoring DNA polymerase activity. Additionally, we illustrate the direct sensing of biological fluids, exemplified by the analysis of urine, while also shedding light on a unique particle assembly process that occurs on LSPR chips. We not only discuss the significance of LSPR sensing but also explore its potential to investigate a plethora of phenomena at liquid–liquid and solid–liquid interfaces. This is particularly noteworthy, as existing transduction methods and sensors fall short in fully comprehending these interfacial phenomena. Concluding our discussion, we present a futuristic perspective that provides insights into potential opportunities emerging at the intersection of fluidics and LSPR sensing.


■ INTRODUCTION
The discovery of localized surface plasmon resonance (LSPR) has laid the cornerstone for multifarious research domains, spanning from material science to biosensing, as documented in prior works. 1,2LSPR is considered as a transduction platform, holding the promise of facilitating the development of costeffective and portable devices for a wide array of applications, as highlighted by recent studies. 3,4LSPR, fundamentally an optical phenomenon, manifests as a synchronized oscillation of valence electrons within noble metals, leading to the absorption of light within the ultraviolet−visible (UV−vis) spectrum.This phenomenon arises from interactions between incident photons and the conduction band of a noble metal nanostructure.An exponentially decaying length of the electromagnetic field is also observed in LSPR, which is on the order of 5−10 nm. 1,5This short decay length in the LSPR is sensitive to the interference caused by fluctuations in the refractive index of the solution in close proximity to the surface (5−10 nm).This unique feature forms the basis for applications in refractive-index-based LSPR bio/chemical sensing, opening avenues for innovative research and technological advancements in the biosensing field.
From classical physics perspective, LSPR can be modeled by Mie's solution to Maxwell's equation, yielding an empirical expression for extinction, E(λ), which is dependent on the dimension, shape, density, and local environment of the nanostructure: 5 E N a ( ) 24 ln(10) ( ) In eq 1, N A represents the areal density of the nanostructure, a denotes the average radius of the nanostructure (modeled as a sphere), λ is the wavelength of the absorbing radiation, ε m stands for the dielectric constant of the medium enveloping the nanostructure (assumed to be a positive, real integer that is independent of λ), ε i signifies the imaginary component of the nanostructure's dielectric function, ε r denotes the real component of the nanostructure's dielectric function, and χ is the term employed to characterize the aspect ratio of the nanostructure.Note that the value of a should be less than λ (absorbed by the nanostructure).
The influence of the local change in the refractive index around the nanostructure on λ can be assessed through absorption spectroscopy, such as by employing simple ultraviolet−visible−infrared (UV−vis−NIR) spectroscopy methods, particularly if the nanostructure absorbs light within the UV− vis−NIR wavelength range.The changes in the wavelength can be described by where m is the bulk refractive index response of the nanostructure, d is the effective adsorbate layer thickness and l d is the characteristic electromagnetic field decay length.It should be noted that the refractive index n is a function of dielectric constant, ε, n = ε 1/2 .Equation 2 shows that changes in the peak wavelength Δλ are directly proportional to Δn, i.e., the local change in the refractive index around the nanostructure.This concept, particularly highlighted by eq 2, serves as the cornerstone of LSPR biosensing, wherein variations in wavelength are directly linked to the binding of biomolecules and analytes (such as cells, proteins, DNA, etc.) onto the nanostructure's surface. 14Different nanoarchitectures have been developed to enhance the sensitivity of the LSPR biosensors, i.e., to maximize the Δλ upon interaction of biomolecules with nanostructures.−24   also shares some of these LSPR substrates, which are combined with microfluidics to form bioassay chips.
−27 These efforts culminated in the creation of extensive nanostructured surfaces, capable of accommodating various biomolecules, ranging from prokaryotic and eukaryotic cells to amino acid-and nucleic acid-based biomolecules. 8,9An essential aspect of our research has been the exploration of the nanomushroom (NM) architecture, 10 which has yielded significant results, particularly in establishing a platform for long-term cell monitoring. 8Previously, nanostructures were known to induce toxic effects. 28−31 Consequently, our developed LSPR platforms, especially NM-based substrates, provide a more conducive environment for cell proliferation, overcoming the previous toxicity challenges.Furthermore, we integrated the NM architecture with microfluidics to achieve high sensitivity in single nucleic acid studies. 32e have also developed portable systems and integrated LSPR with semiconductor technology, facilitating the transition of LSPR technology from academia to industry. 33,34CREATING AN INTEGRATED LSPR CHIP WITH

MICROFLUIDICS
In this section, we will delve into our past experiences in developing a simple LSPR chip and its integration with microfluidics. 35Additionally, we will explore potential avenues for refining the fabrication process.The following is a step-bystep protocol for creating an LSPR microfluidic chip: 1. Selecting the ideal substrate for metal nanostructure fabrication: Choose from standard options such as borosilicate glass slides, pure silicon dioxide (SiO 2 ), silicon (Si), or silicon-based materials like silicon carbide or silicon nitride as dielectric substrates for crafting LSPRactive nanostructures.It is important to emphasize that the substrate choice significantly influences sensitivity, as demonstrated in our previous studies. 36

Substrate cleaning:
The selected substrate should be thoroughly cleaned.For instance, when working with pure silicon dioxide (SiO 2 ) and silicon (Si), we employed the RCA (Radio Corporation of America) protocol, detailed as follows: in the heated solution for 10 s to effectively eliminate inorganic contaminants.After this step, thoroughly rinse the substrate with DI water to complete the cleaning process, making it ready for its intended use.Note that many glass and silicon manufacturers now provide substrates that are already sufficiently clean, sometimes even polishing them before shipping.In such cases, the RCA cleaning protocol can be substituted with a milder cleaning method using acetone and isopropanol (IPA) as the cleaning agents. 39Initially, immerse the substrate in acetone and sonicate it for 5 min.Follow this with a thorough rinse using IPA to remove any residues or stains from the acetone cleaning process. 40. Thin f ilm deposition: After completing the cleaning process, apply a gold film with a thickness of less than 5 nm to the meticulously cleaned substrate.It is important to note that the thickness of this gold film directly impacts the size and spacing of the resulting nanostructures.While thicknesses up to 10 nm can yield nanoparticles, 41 for optimal sensitivity when creating NM structures, we recommend a film thickness of up to 5 nm. 4. High-temperature annealing: The selected substrate, featuring a deposited thin film of gold, undergoes annealing at a temperature of 560 °C for a duration of 3.5 h within a hot furnace.Note that the temperature of annealing is different for different substrates/LSPR active material as it mainly depends on the critical temperature of the material with which the chip is developed.Following this annealing process, plasmonic gold nanoislands form on the substrate, displaying a distinctive pinkish coloration.At this stage, we have successfully developed a nanoisland (NI) LSPR chip. 5. Etching: The NI LSPR chip is now exposed to inductively coupled plasma reactive ion etching (ICP-RIE) technique with SF 6 gas to convert the nanoisland (NI) into nanomushroom (NM) configuration.In our specific study, the ICP-RIE instrument's internal chamber pressure was set to 10 mTorr, with a flow rate of 45 sccm (standard cubic centimeters per minute) for SF 6 .Finally, assembly of the LSPR microfluidic chip is completed.The detailed fabrication steps for creating the PDMS template with microfluidic channels and its subsequent bonding to the LSPR chip are visually explained in Figure 2.
It should be noted that PDMS can also be replaced with PMMA (poly(methyl methacrylate)) to fabricate microchannels.Fabriating microfluidic−nanoplasmonic devices with PMMA entails the design of micropatterns using AutoCAD, followed by the printing of layers onto a PMMA substrate using a laser cutter or micromilling machine and subsequently assembling them to form a fully functional microfluidic device.

■ BIOSENSING USING MICROFLUIDIC LSPR CHIPS
Here, we present a microfluidic LSPR chip designed for realtime, label-free DNA molecule detection and analysis. 11The microfluidic chip is initially constructed with a PDMS microfluidic channel, which is subsequently bonded to the LSPR chip.This LSPR chip is characterized by NM nanostructures, comprising silicon dioxide stems (40 nm) and gold caps (22 nm), with an average spacing of 19 nm.To enable effective spacing and surface distribution of specific singlestranded DNA (ssDNA), denoted as T30, on the LSPR substrate, hexanedithiol (HDT) molecules are introduced in a 1:1 molar ratio with the DNA.This prepares the substrate for subsequent molecular interactions.A DNA primer, identified as P8, is then attached to the existing DNA strands on the LSPR chip.The crucial step involves the action of DNA polymerase, an enzyme catalyzing the elongation of DNA strands.This process involves the sequential assembly of nucleotides from the surrounding fluid, effectively creating a complementary DNA strand.Further details regarding the sensing mechanism and sensor response are elucidated in Figure 3 and Figure 4.
Importantly, all of these molecular interactions and reactions occur within the confines of the microfluidic LSPR chip.The real-time nature of this detection method enables researchers to observe and monitor each step of the intricate reaction as it unfolds, all at room temperature.For instances, from results shared in Figure 4, we can observe a shift of 4.19 ± 0.48 nm upon polymerization of double-stranded DNA.These wavelength shifts are in close agreement with the theoretical expected wavelength shifts of 3.24 nm for a LSPR sensor with a sensitively of 54 ± 6 nm, when used for interrogation of 0.06 units change in refractive index.Insights into dynamics of individual steps of primer attachment, elongation by polymerase, and its release can also be observed in the real time (Figure 4).However, without additional extensive validation the reaction steps using other complementary techniques (such as quartz crystal microbalance) or testing other DNA systems, only qualitative conclusions related to the completion of the reaction can be drawn from the results.It should also be noted that the wavelength shifts are computed for each reaction step relative to the signal change observed in the preceding step.The overall wavelength shifts throughout the reaction are determined relative to the beginning of the reaction. 11This detection process is entirely label-free, eliminating the need for external markers or labels.Ultimately, we establish a correlation between the sensor response and the quantities of DNA and HDT molecules immobilized on the surface.This correlation allows for a comprehensive and detailed analysis of DNA-related reactions, providing valuable insights into molecular-level processes through the microfluidic LSPR chip.A general qualitative comparison of LSPR-based microfludic chip versus the other microfluidic chips in the literature is also shared in Table 2.

■ SPECIFIC GRAVITY SENSING OF BIOFLUIDS
The measurement of biofluid specific gravity is a critical parameter that can be directly addressed by using LSPR chips.−44 Specific gravity assessment aids in evaluating the concentration of solutes in these fluids, offering insights into hydration levels, kidney function, and potential disorders.In the case of sputum, specific gravity analysis aids in the evaluation of respiratory conditions. 45Meanwhile, deviations observed in blood plasma and serum specific gravity may signal dehydration or abnormal solute concentrations, assisting in the diagnosis of metabolic disorders or renal issues. 46n our research, we have conducted specific gravity measurements of urine, covering a range from 1.005 to 1.043. 47These measurements were made using real human urine samples in an experiment where urine was collected from the donor before and after running a 10 km distance (to assess the hydration conditions).The collected urine was then subjected to the LSPR chip made of gold nanoislands, and we observed a red-shift when the darker color urine (higher specific gravity) was tested using the LSPR chip.The sensitivities of the chip were found to be 79.21 nm per urine specific gravity unit.Further details are provided in Figure 5.This precise measurement of specific gravity serves as a reliable indicator for assessing hydration levels, with variations indicating potential dehydration or overhydration.Furthermore, it plays a crucial role in evaluating kidney function, as deviations in specific gravity may reveal renal insufficiency.Beyond that, this diagnostic parameter proves invaluable in the monitoring and early detection of urinary tract infections, kidney stones, and fluid retention, facilitating prompt intervention and tailored treatment plans. 48rom the sensor's perspective, the LSPR sensor offers a distinct advantage over traditional specific gravity measurement tools like refractometers, which most rely on subjective observations susceptible to variation among individuals (a scale is read by the human eye; see Figure 6, showing the evolution of a traditional specific gravity measurement scale to a fluidic sensor).While there are refractometers described in the literature 49 that do not rely on direct visual reading of measurements, such sensor types (compared to traditional refractometers) have typically not exclusively found clinical applications such as for urine specific gravity measurement.LSPR mitigates such potential errors or discrepancies by directly displaying sensor responses, eliminating the reliance on human interpretation.Beyond its sensitivity and selectivity, LSPR sensors provide the added benefit of easy storage or transmission of data to a database.This facilitates advanced functionalities, including its integration with artificial intelligence, enhancing the overall efficiency and capabilities of the measurement system.■ DETERMINISTIC PARTICLE ASSEMBLY ON LSPR

CHIPS: TOWARD PUMPLESS FLOW CONTROL
Controlled particle assembly plays a pivotal role in both materials science and analytical biology. 50,51Accurate particle arrangements result in advanced materials that exhibit customized optical and electronic properties.In the realm of analytical biology, this capability enables the assembly of bioentities, serving a multitude of scientific purposes, including biosensing and the development of organ-on-chip systems. 52,53n our research, we have successfully demonstrated particle assembly using LSPR chips for micrometer-sized silica particles. 32This process is initiated by the formation of laserinduced hot spots on LSPR chips, resulting in the generation of microbubbles that guide the assembly of the particles.While thermoplasmonic flow and microbubble formation are observed with SPR substrates as well, it is important to note that the exclusive capability of particle assembly is unique to LSPR chips.This distinction is attributed to the presence of nanoscale air gaps and the dissipation of electron energy within LSPR nanostructures.By carefully adjusting parameters, such as laser power, circular laser sequence radius, and particle concentration, we gain precise control over the particle count and the assembled structure.This framework firmly establishes LSPR chips as a versatile platform for light-driven particle assembly, with further insights provided in Figure 7.
The formation of clusters can be elucidated by the solvent evaporation process induced by heating the nanostructures with the laser.This heating generates a temperature gradient within the liquid as the bubble expands.Consequently, the growing bubble induces a convective flow of fluid toward itself, effectively bringing the particles closer to the surface of the microbubble.When we disregard inertia and gravitational effects, the motion of these particles can be described by eq 3:  where F D is the drag force, F T is the thermophoretic force, and F B is the Brownian force.In our system, F D dominates the other two forces.The Peclet number, the ratio between flow and Brownian time scales, is on the order of 1 × 10 2 for our experiments.Additionally, the thermophoretic velocity is 10 −6 m/s, which is 1 order of magnitude lower than the translational velocity (2 × 10 −5 m/s) of the particles in our experiments.When particles reach the vicinity of the bubble, they are primarily influenced by the dominant drag force, which notably does not cause the bubble to collapse.This intriguing phenomenon can be attributed to the favorable ratio between the drag force generated by particle velocity and the gravitational force acting on the particles, roughly on the order of 1 × 10 −1 .Consequently, the particles tend to accumulate at the contact line between the substrate and the bubble interface.Once pinned at this interface, these particles accompany the bubble during the condensation process and ultimately coalesce into a cluster when the bubble eventually collapses.The self-assembly process is visually depicted in Figure 8, illustrating the gradual decay of the bubble radius upon laser removal.
One intriguing application of this concept involves the exploration of cell−cell interactions within a microfluidic device, obviating the need for an external pump. 54,55In this scenario, cells suspended in a solution could be strategically assembled and transported from one location on a chip to another by using a microbubble.This process facilitates the mixing of cells with other biomolecules present on the chip before the bubble ultimately collapses.The microbubble's specific density could also enable it to selectively lift particular cells and transport them to different locations on the chip.Notably, since the substrate is plasmonic, the same laser used for bubble generation and transport can also serve as a tool for detecting LSPR signatures or changes in refractive index resulting from the assembled species.This dual-purpose utilization of the laser enables both sensing and pump-free fluid manipulation, encompassing activities such as fluid mixing and transport.
With regard to application, the integration of particle assembly techniques with LSPR chips presents a new opportunity in bio/chemical sensing capabilities.For instance, by leveraging precise control over the particle arrangement, researchers can tailor sensing platforms with superior sensitivity, selectivity, and dynamic range.This approach enables the development of multiplexed sensing systems capable of the simultaneous detection of multiple analytes within a single assay, expanding the scope of applications in fields such as medical diagnostics, environmental monitoring, and food safety.Moreover, the dynamic nature of particle assembly on LSPR chips facilitates real-time modulation of sensor performance in response to environmental cues or analyte interactions, enabling on-demand tuning of the sensitivity and specificity.Integration with microfluidic systems further enhances sensor functionality by enabling pumpless flow control and precise sample manipulation, leading to improved mass transport kinetics and reduced sample volumes.Together, these advancements herald a new era of biosensing technology, where the synergy between particle assembly and LSPR chips unlocks novel possibilities for the development of versatile, high-performance sensors with broad-ranging applications in biomedical, environmental, and industrial settings.−58 This sensitivity stems from LSPR's ability to detect changes in the refractive index of nanostructures situated within distances of 5−10 nm from the surface.This specific range aligns with the realm where interfacial effects hold significant sway, rendering the LSPR highly relevant for exploring a wide array of applications.These applications span from gaining insights into fluid rheology to detecting complex phenomena like liquid slips. 59,60SPR sensors offer a valuable means to explore various liquid−liquid interactions, with emulsification serving as a prominent example. 61,62Emulsification is the process of dispersing one liquid into another in the form of small droplets and is encountered in everyday products such as salad dressing and mayonnaise, which are typical oil-in-water type emulsions.It is worth noting that certain surfactants, such as egg in mayonnaise, are used as stabilizers in such mixtures to reduce interfacial tension between water and oil components.
Leveraging LSPR sensor sensitivity to changes in the refractive index at the oil−water interface can be invaluable for scrutinizing the dynamic interplay between these substances, offering potential insights into optimizing interfacial tension and, consequently, refining taste.Additionally, LSPR can facilitate the study of fluid parameters crucial for interfacial rheology, including the density and viscosity of the liquid−liquid interface. 63,64This is due to the refractive index's dependence on factors like density and viscosity in the context of the liquid− liquid interface.
The versatility of LSPR can extend to examining flow effects, such as Marangoni effects, where variations in surface tension induce fluid flow, resulting in the movement of one liquid over the other. 65This offers an opportunity to investigate two vital parameters: the refractive index of moving fluids compared with stationary liquids and the quantification of surface tension using optical signatures of plasmons at the liquid−liquid interface.LSPR methods can also provide a useful platform for exploring the complexities of viscoelastic flow instabilities and elastic turbulence. 66,67xploring solid−liquid interfaces is crucial for a comprehensive understanding of fluid flows, demanding surface-sensitive experimental techniques such as atomic force microscopy and nonlinear optical methods.In this context, LSPR provides a unique opportunity to investigate solid−liquid interfaces through high-frequency resonating nanostructures.The configuration of LSPR nanostructures can be adjusted to create surfaces with different wettabilities, ranging from hydrophobic to hydrophilic features.These varied wetting conditions on solid surfaces offer a distinctive platform for fluid interactions, leading to the generation of friction forces, shear stress, and liquid slips at the solid−liquid interface. 68,69These forces directly influence the properties of the nanostructures, which can be elucidated through absorption spectroscopy to identify the associated LSPR resonances.Another area of investigation is the bacteria biofilms. 9,69LSPR can be useful to investigate dynamics of how the biofilm interacts with the solid surfaces, revealing properties such as adhesive forces and viscoelastic properties of the film.Some other applications where LSPR can be extended in the near future are also shared in Figure 9.

■ CONCLUSIONS AND FUTURE PROSPECTS
The future of LSPR sensors promises significant advancements across diverse fluidic domains, particularly in the interplay between electrical plasmons and fluidic interfaces.This emerging area holds great potential for developing novel measurement techniques at the solid/liquid interface and uncovering new scientific insights.To fully realize this potential, further research is needed to establish a synergy between electromagnetic fields and fluidic environments.
One of the central challenges for future LSPR sensing lies in designing sensor chips with enhanced sensitivity and stability.Advancements in chip technology are essential to ensure precise and reliable measurements in dynamic and complex fluidic environments.This necessitates the refining of the structural and material composition of LSPR chips, potentially leading to more selective sensors.Additionally, the discovery and integration of new materials will expand the capabilities of LSPR sensors, allowing for tailored designs to optimize the performance in various fluid dynamics experiments.
In the realm of bio/chemical sensing, a deeper understanding of how LSPR sensors interact with biological entities is imperative.Isolating the effects of fluid flow is crucial for accurate interpretation in sensing applications, leading to more precise and reliable biosensor responses, with applications spanning medical diagnostics to environmental monitoring.
The future of LSPR sensing will also witness a surge in sophisticated simulation techniques, enabling researchers to model and predict LSPR behaviors under diverse fluid flow conditions.This predictive capability will provide valuable insights into sensor responses before experimental implementation and results interpretation.
Efficiently transferring LSPR sensor technology from research settings to practical applications is paramount.Bridging the gap between academia and industry will facilitate the integration of LSPR sensors into real-world scenarios ranging from healthcare to industrial processes.While miniaturization has enabled portable LSPR sensors, striking the right balance between sensitivity and portability remains crucial.Future developments should aim to optimize sensor size without compromising performance, particularly for on-site and field applications, where portability is a key consideration.
In summary, the future of LSPR sensors will be characterized by interdisciplinary collaborations, technological breakthroughs, and a deeper understanding of the interplay among fluid physics, interfacial science, materials, and biological entities.As researchers continue to push the boundaries of LSPR technology, its transformative impact on diverse fields will become increasingly evident, opening new avenues for exploration and innovation.

Figure 1 .
Figure 1.Fabrication of nanomushroom (NM) arrays in SF 6 plasma.(a) Scanning electron microscopy (SEM) image and schematic representation of nanoislands (NI).(b) Schematic depicting an SiO 2 substrate with NI inside an SF 6 plasma chamber, where SF x reactive ions etch out both SiO 2 and Au.(c) SEM image and schematic of NM formed after the NI being exposed to reactive ionetching (RIE) inside SF 6 plasma.The inset shows the cross section of the NM.The developed NM structures are 45−60 nm in total height with an average spacing of 7.96 ± 2.12 nm.(d−g) Examples of various configurations of NM substrates or integrated chips: (d) NM-coated glass slide of 2.5 cm × 7.5 cm (the pink color corresponds to the Au nanostructures); (e) spots of 3 mm circles for multiplex bioassay applications; (f) integrated polydimethylsiloxane (PDMS) wells on an NM substrate; (g) sealed device with PDMS microfluidic channels on an NM substrate connected with liquid delivery tubings.Reproduced with permission from ref 10.

Figure 2 .
Figure 2. Soft lithography steps: (a) Si substrate, (b) coating of photoresist on Si substrate, (c) exposure of photoresist to micropatterns, and (d) transfer of micropatterns onto the photoresist.(e) Development of the master mold.(f) and (g) represent the transfer of micropatterns from the mold to PDMS, and (h) LSPR and PDMS substrates ready for bonding.Reproduced with permission from ref 35.Copyright 2018 Elsevier.

Figure 3 .
Figure 3. LSPR DNA sensing microfluidic setup (A) Reaction scheme on a gold (Au) LSPR substrate, involving an immobilized ssDNA template (T30) with HDT, addition of primer sequence P8, and Klenow fragment of DNA polymerase along with dNTPs.Polymerase catalyzes the formation of the complementary DNA strand by assembling dNTPs from the surrounding media.(B) Snapshots of a LSPR microfluidic chip, in operation with indented reflection probe (i) and without (ii).In both cases the fluid inlet reservoir and the outlet tubing are shown, and (C) shows a schematic of the microfluidic nanoplasmonic chip consisting of the bottom nanoplasmonic substrate, a PDMS, and a poly(methyl methacrylate) (PMMA) substrate.Reproduced with permission from ref 11.Copyright 2019 Elsevier.

Figure 4 .
Figure 4. Label-free real-time DNA/HDT immobilization and polymerase activity monitoring using LSPR measurements.(a) Real-time sensogram showing the shift in the maximum wavelength of the reflected light during immobilization of DNA and HDT, primer binding, DNA elongation, and intermediate washing steps.(b) Sample reflection spectra of bare microfluidic chip and the chip with ds30-mer showing a total wavelength shift of 2.7 nm. (c) Mean wavelength shifts from each step, calculated from 6 polymerase reactions and 3 controls (no polymerase and no dNTPs) experiments.Error bars represent the standard error of the mean.The polymerase versus "no dNTP" is significant with p < 0.05.Reproduced with permission from ref 11.Copyright 2019 Elsevier.

Figure 5 .
Figure 5. Urine specific gravity measurement.(a) Three individuals exhibiting distinct hydration levels, each reflected in varying urine specific gravity.(b) Schematic representation of the LSPR measurement process.(c) LSPR spectra, showcasing absorbance and wavelength shifts corresponding to changes in urine specific gravity.Adapted with permission under a Creative Commons CC-BY 4.0 license from ref 47.Copyright 2024 Wiley.

Figure 6 .
Figure 6.Scale to fluidic sensor: (a) Schematic of a refractive index scale for the measurement of urine parameters which involves observation using the human eye.The schematic also shows a urine collection tube.The arrows indicate the location of the scale (drawn not to the scale) and the location where urine is added on the refractometer.(b) Part i shows a picture of the LSPR sensors, where the length of the scale is 1 cm.Part ii shows a scanning electron microscopy image of the LSPR sensor surface.Part iii shows measurement of the schematic.Part iv reveals the mean diameter and spacing of the nanoislands on the LSPR sensor surface.Reprinted with permission under a Creative Commons CC-BY 4.0 license from ref 47.Copyright 2024 Wiley.

Figure 8 .
Figure 8. Decay of the bubble radius upon the laser removal.The pictures are captured at (a) 1, (b) 44, (c) 71, (d) 76, (e) 79, and (f) 82 s after removing the laser beam.The scale bar is 20 μm.The concentration of the dispersion and the size of the laser sequence used in these experiments were 0.04 g/L and 10 μm, respectively.Reproduced with permission from ref 32.Copyright 2021 Elsevier.
Nanotechnology and Integrated Bioengineering Centre (NIBEC), School of Engineering and Healthcare Technology Hub, School of Engineering, Ulster University, Belfast BT15 1AP, United Kingdom; orcid.org/0000-0002-4720-3679;Email: n.bhalla@ulster.ac.ukAmy Q. Shen − Micro/Bio/Nanofluidics Unit, Okinawa Institute of Science and Technology Graduate Univerisity, Onna-son, Okinawa 904-0495, Japan; orcid.org/0000-0002-1222-6264;Email: amy.shen@osit.jpComplete contact information is available at: https://pubs.acs.org/10.1021/acs.langmuir.4c00374Notes The authors declare no competing financial interest.Biographies Dr. Nikhil Bhalla is currently working as a Lecturer (Assistant Professor) in the School of Engineering at the University of Ulster, UK.His research is in the area of biosensing with a prime focus on nanoplasmonic technologies (such as LSPR) and interfacial science of biosensing materials, transducers and biorecognition layers.He focuses on use of his developed sensors for healthcare and food applications.Prior to his independent position, he worked as a postdoctoral scholar in OIST, Japan, with Professor Amy Shen after receiving his PhD from the Department of Electronic and Electrical Engineering, University of Bath, UK, in 2016.He also has a first division honours degree (B.E.Hons) in Electronics and Instrumentation Engineering from BITS-Pilani, India, and a M.S. in Microelectronics and Applications from CYCU, Taiwan.Dr. Amy Shen, the Provost and a Professor at the Okinawa Institute of Science and Technology Graduate University (OIST) in Japan, leads the iMicro/Bio/Nanofluidics Unit.Her research primarily focuses on advancing microfluidics and lab-on-a-chip technologies at the bio/ nanointerface, with significant applications in biotechnology.Recognized as a Fellow by the American Physical Society, the Royal Society of Chemistry, and the Society of Rheology, and honored as a Fulbright Scholar in 2013, Dr. Shen has made substantial contributions to the field of microfluidics and rheology.She also actively contributes to the scientific community as an associate editor for Soft Matter and serves on the editorial advisory boards for ACS Sensors, Journal of Rheology, and Physics of Fluids, highlighting her role in guiding pivotal research and innovation in microfluidics and biosensors.

Table 1 .
Different Nanoarchitectures Developed for LSPR-Based Microfluidics Bioassay Systems 37,38(a) Removal of organic contaminants: Prepare a solution with a 5:1:1 ratio of H 2 O:NH 4 OH: H 2 O 2 .To prepare this solution, start by adding NH 4 OH to deionized (DI) water.Heat the resulting mixture on a hot plate at 80 °C until noticeable bubbles form.At this point, introduce H 2 O 2 , which will significantly increase the bubble formation within

Table 2 .
Qualitative Comparison of LSPR-Based Microfluidic Chips versus Other Microfluidic Chips