Patterned resonance plasmonic microarrays for high-performance SPR imaging

Direct detection of virus-like particles using color images of plasmonic nanostructures

We propose a measurement method for sensitive and label-free detections of virus-like particles (VLPs) using color images of nanoplasmonic sensing chips. The nanoplasmonic chip consists of 5×5 gold nanoslit arrays and the gold surface is modified with specific antibodies for spike protein. The resonant wavelength of the 430-nm-period gold nanoslit arrays underwater environment is about 570 nm which falls between the green and red bands of the color CCD. The captured VLPs by the specific antibodies shift the plasmonic resonance of the gold nanoslits. It results in an increased brightness of green pixels and decreased brightness of red pixels. The image contrast signals of (green - red) / (red + green) show good linearity with the surface particle density. The experimental tests show the image contrast method can detect 100-nm polystyrene particles with a surface density smaller than 2 particles/µm². We demonstrate the application for direct detection of SARS-CoV-2 VLPs using a simple scanner platform. A detection limit smaller than 1 pg/mL with a detection time less than 30 minutes can be achieved.

Probing Herbicide Toxicity to Algae (Selenastrum capricornutum) by Lipid Profiling with Machine Learning and Microchip/MALDI-TOF Mass Spectrometry

Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS)-based lipid profiling is a powerful method to study the cytotoxicity of chemical exposure to microorganisms at the single cell level. We report here a combined approach of machine learning (ML) and microchip-based MALDI-time of flight (TOF) mass spectrometry to investigate the cytotoxic effect of herbicides on algae through single cell lipid profiling. Algal species Selenastrum capricornutum was chosen as the target system, and its exposure to different doses of common chemical herbicides and the resulting cytotoxic behaviors under various stress conditions were characterized. A lipid library for S. capricornutum has been established with 63 identified lipids that include glycosyldiacylglycerols and triacylglycerols. We demonstrated that major alternations occurred for lipids with functional groups of digalactosyldiacylglycerol (DGDG), triacylglycerol (TAG), and monogalactosyldiacylglycerol (MGDG). DGDG was shown to decrease upon exposure to herbicides of norflurazon and atrazine, while some MGDG and TAG lipids would increase for norflurazon. Compared to other algae, S. capricornutum was more strongly impacted by norflurazon than atrazine while the latter was observed to have a greater effect on C. reinhardtii. Machine learning algorithms have been applied to improve the classification of herbicide impact and help identify lipid species affected by the chemical exposure. A total of 69 machine learning models were trained and tested for the identification of ideal algorithms in the classification process, in which flexible discriminant analysis and support vector machine model were found to be the most accurate and consistent. The ML algorithms accurately differentiated herbicide impact and have identified cytotoxic differences that were previously hidden. The results suggest that herbicides express toxicity among different algae likely on the basis of metabolic differences. The ML-assisted method proves to be highly effective and can provide an advanced technological platform for probing cytotoxicity for bacterial species and in metabolic pathway analysis.

Lipidomic Profiling of Algae with Microarray MALDI-MS toward Ecotoxicological Monitoring of Herbicide Exposure

Misuse of agrochemicals has a long-lasting negative impact on aquatic systems. Mismanagement of herbicides in agri-food sectors is often linked to a simultaneous decline in the health of downstream waterways. However, monitoring the herbicide levels in these areas is a laborious task, and modern analytical approaches, such as solid-phase extraction-liquid chromatography-mass spectrometry (SPE-LC-MS) and enzyme-linked immunosorbent assay, are low-throughput and require significant sample preparation. We report here the use of microchip technology in combination with matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) for the assessment of the ecotoxicological effect of agrochemicals on aquatic species at the single-cell level. This approach quantifies the fluctuations in lipid content in sentinel organisms and targets the microalga, Chlamydomonas reinhardtii (C. reinhardtii), as the model system. Specifically, we investigated the cytotoxicity of three herbicides (atrazine, clomazone, and norflurazon) on C. reinhardtii by analyzing the lipid component variation upon assorted herbicide exposure. Lipidomic profiling reveals a significantly altered lipid content at >EC₅₀ in atrazine-exposed cells. The response for norflurazon showed similar trends but diminished in magnitude, while the result for clomazone was near muted. At lower herbicide concentrations, digalactosyldiacylglycerols showed a rapid decrease in abundance, while several other lipids displayed a moderate increase. The microchip-based MALDI technique demonstrates the ability to achieve lipidomic profiling of aquatic species exposed to different stressors, proving effective for high-throughput screening and single-cell analysis in ecotoxicity studies.

Detection of Multiple Sclerosis Biomarkers in Serum by Ganglioside Microarrays and Surface Plasmon Resonance Imaging

Multiple sclerosis (MS) is an autoimmune disease that damages the myelin sheaths of nerve cells in the central nervous system. An individual suffering from MS produces increased levels of antibodies that target cell membrane components, such as phospholipids, gangliosides, and membrane proteins. Among them, anti-ganglioside antibodies are considered as important biomarkers to differentiate MS from other diseases that exhibit similar symptoms. We report here a label-free method for detecting a series of antibodies against gangliosides in serum by surface plasmon resonance imaging (SPRi) in combination with a carbohydrate microarray. The ganglioside array was fabricated with a plasmonically tuned, background-free biochip, and coated with a perfluorodecyltrichlorosilane (PFDTS) layer for antigen attachment as a self-assembled pseudo-myelin sheath. The chip was characterized with AFM and matrix-assisted laser desorption ionization mass spectrometry, demonstrating effective functionalization of the surface. SPRi measurements of patients' mimicking blood samples were conducted. A multiplexed detection of antibodies for anti-GT_1b, anti-GM₁, and anti-GA₁ in serum was demonstrated, with a working range of 1 to 100 ng/mL, suggesting that it is well suited for clinical assessment of antibody abnormality in MS patients. Statistical analyses, including PLS-DA and PCA show the array allows comprehensive characterization of cross reactivity patterns between the MS specific antibodies and can generate a wide range of information compared to traditional end point assays. This work uses PFDTS surface functionalization and enables direct MS biomarker detection in serum, offering a powerful alternative for MS assessment and potentially improved patient care.

A microwell array structured surface plasmon resonance imaging gold chip for high-performance label-free immunoassay

Surface plasmon resonance imaging (SPRi) offers a compelling method for high-throughput, real-time, and label-free biomolecular interaction studies and immunoassays, but its performance suffers from limited intrinsic sensitivity and low-contrast SPRi images. Herein we report a high-performance SPRi chip featuring patterned microwell array constructed by photolithography of adhesive polydopamine (PDA) thin film on conventional gold chip. The chip allows for the facile construction of region-defined sensing array on its surface with improved intrinsic SPRi sensitivity due to the intensified surface plasmon wave (SPW) in the microwells. The immunoassay performance of the as-designed SPRi chip is evaluated by using anti-ochratoxin A (anti-OTA) monoclonal antibody as a model target. The results show that this microwell array structured gold chip exhibits ca. 18%-32% higher signal intensity than the conventional gold chip when detecting anti-OTA at different concentrations, and the noise remains at the same level, showing enhanced intrinsic sensitivity. Meanwhile, this microwell-structured chip affords clear and high-contrast SPRi images with well-defined sensing areas, which greatly facilitates the extraction and quantitative analysis of detection signals while efficiently suppressing the disturbance from background areas.

Plasmonic Biosensing with Aluminum Thin Films under the Kretschmann Configuration

Aluminum has recently attracted considerable interest as a plasmonic material due to its unique optical properties, but most work has been limited to nanostructures. We report here SPR biosensing with aluminum thin-films using the standard Kretschmann configuration that has previously been dominated by gold films. Electron-beam physical vapor deposition (EBPVD)-prepared Al films oxidize in air to form a nanofilm of Al₂O_3, yielding robust stability for sensing applications in buffered solutions. FDTD simulations revealed a sharp plasmonic dip in the visible range that enables measurement of both angular shift and reflection intensity change at a fixed angle. Bulk and surface tests indicated that Al films exhibited superb sensitivity performance in both categories. Compared to Au, the Al/Al₂O₃ layer showed a marked effect of suppressing nonspecific binding from proteins in human serum. Further characterization indicated that Al film demonstrated a higher sensitivity and a wider working range than Au films when used for SPR imaging analysis. Combined with its economic and manufacturing benefits, the Al thin-film has the potential to become a highly advantageous plasmonic substrate to meet a wide range of biosensing needs in SPR configurations.

Ultrasensitive Detection of Bacterial Protein Toxins on Patterned Microarray via Surface Plasmon Resonance Imaging with Signal Amplification by Conjugate Nanoparticle Clusters

Sensitive detection and monitoring of biological interactions in a high throughput, multiplexed array format has numerous advantages. We report here a method to enhance detection sensitivity in surface plasmon resonance (SPR) spectroscopy and SPR imaging via the effect of accumulation of conjugated nanoparticles of varying sizes. Bacterial cholera toxin (CT) was chosen for the demonstration of enhanced immunoassay by SPR. After immobilization of CT on a gold surface, specific recognition is achieved by biotinylated anti-CT. The signal is amplified by the attachment of biotinylated 20 nm AuNP via streptavidin bridge, followed by attachment of 5 nm streptavidin-functionalized Fe₃O₄NP to the AuNP-biotin surface. The continuous surface binding of two differently sized conjugated nanoparticles effectively increases their packing density on surface and significantly improves SPR detection sensitivity, allowing quantitative measurement of CT at very low concentration. The dense packing of conjugated nanoparticles on the surface was confirmed by atomic force microscopy characterization. SPR imaging of the immunoassay for high-throughput analysis utilized an Au-well microarray that attenuated the background resonance interference on the resulting images. A calibration curve of conjugated nanoparticle binding signal amplification for CT detection based on surface coverage has been obtained that shows a correlation in a range from 6.31 × 10^-16 to 2.51 × 10^-13 mol/cm² with the limit of detection of 5.01 × 10^-16 mol/cm². The absolute quantity of detection limit using SPR imaging was 0.25 fmol. The versatile nanoparticles and biotin-streptavidin interaction used here should allow adaptation of this enhancement method to many other systems that include DNA, RNA, peptides, and carbohydrates, opening new avenues for ultrasensitive analysis of biomolecules.

Gold Nanostar Enhanced Surface Plasmon Resonance Detection of an Antibiotic at Attomolar Concentrations via an Aptamer-Antibody Sandwich Assay

A new sandwich assay for tetracycline (TC) involving a DNA aptamer and antibody pair is demonstrated in conjunction with gold nanostar (GNS) enhanced surface plasmon resonance (SPR) to achieve detection in the low attomolar range. GNS particles were covalently functionalized with the antibody probe (antiTC) and integrated into a surface sandwich assay in conjunction with a SPR gold chip modified with the TC-specific aptamer. After it was demonstrated that both affinity probes can bind simultaneously to TC, optimization of the assay was performed using either antiTC only or GNS-antiTC conjugates to interact with aptamer/TC complexes present on the chip surface. Target concentrations as low as 10 aM could be detected using GNS-antiTC's, which was >10³ times greater in performance than when using antiTC only. In addition, good selectivity was achieved with respect to other tetracycline derivative antibiotics, such as oxytetracycline (OTC) and chlortetracycline (CTC), both which are structurally similar to TC. As a demonstration of trace antibiotic analysis in environmental samples, the GNS enhanced sandwich assay was applied to analyze TC added to aliquots of local river water and the results validated by comparing to conventional high-performance liquid chromatography (HPLC) analysis.

DNA Linkers and Diluents for Ultrastable Gold Nanoparticle Bioconjugates in Multiplexed Assay Development

A novel bioconjugation strategy leading to ultrastable gold nanoparticles (AuNPs), utilizing DNA linkers and diluents in place of traditional self-assembled monolayers, is reported. The protective capacity of DNA confers straightforward biomolecular attachment and multistep derivatization capabilities to these nanoparticles and, more significantly, substantially enhances their stability in demanding and complex sensing environments. The DNA/AuNPs were assembled through pH-assisted thiol-gold bonding of single stranded DNA and salt aging, with preconjugated biotin moieties facing outward from the gold surface. These nanoparticles remain a stable colloidal suspension under a wide range of buffers and ionic strengths and can endure multiple rounds of lyophilization while retaining high biological activity. Furthermore, the high stability of the DNA/AuNPs allows for multiple reactions and conjugations to be performed within the colloidal suspensions (i.e., Protein A and antibody binding) for tailored and specific recognition to take place. We have demonstrated the applications of the DNA/AuNPs for colorimetric assays and ELISA feasibility; additionally, SPR imaging analysis of a supported membrane microarray shows excellent results with DNA/AuNPs as the enhancing agent. Together, the properties imparted by this interface render the material suitable for clinical and point-of-care applications where stability, throughput, and extended shelf lives are needed.

One-Step Ligand Immobilization and Single Sample Injection for Regeneration-Free Surface Plasmon Resonance Measurements of Biomolecular Interactions

Surface plasmon resonance (SPR) has been well established as a method of choice for label-free kinetic measurements of biomolecular interactions. The conventional approach involves multiple injections of an analyte of different concentrations into a fluidic channel covered with a fixed ligand density. Optimization of the experimental conditions and assessment of the data quality can be complicated by issues such as disruption of the ligand structure by the regeneration step and the limited availability of the sample solution. By sequentially closing fluidic channels on a five-channel SPR instrument, different densities of a ligand can be immobilized and determined in one step. With a subsequent injection of a single sample solution, SPR sensorgrams can be simultaneously collected to yield binding and dissociation rate constants (k_a and k_d) and dissociation constant (K_D) between the ligand and analyte. For biomolecular interactions that obey the Langmuir isotherm, we show that the fidelity of the kinetic data can only be reliably confirmed when there exists a strong linear correlation between the SPR signals and the ligand densities. The use of a multichannel SPR instrument also obviates the regeneration step, allowing the binding kinetics between the green fluorescent protein and its antibody to be measured. In comparison to the conventional approach, the method simplifies the experimental procedure, reduces costs associated with sensor chips and biological samples, expedites kinetic measurements, and allows affinity constants to be determined more straightforwardly.

Femtomolar Detection of Tau Proteins in Undiluted Plasma Using Surface Plasmon Resonance

The ability to directly detect Tau protein and other neurodegenerative biomarkers in human plasma at clinically relevant concentrations continues to be a significant hurdle for the establishment of diagnostic tests for Alzheimer's disease (AD). In this article, we introduce a new DNA aptamer/antibody sandwich assay pairing and apply it for the detection of human Tau 381 in undiluted plasma at concentrations as low as 10 fM. This was achieved on a multichannel surface plasmon resonance (SPR) platform with the challenge of working in plasma overcome through the development of a tailored mixed monolayer surface chemistry. In addition, a robust methodology was developed involving various same chip control measurements on reference channels to which the detection signal was normalized. Comparative measurements in plasma between SPR and enzyme-linked immunosorbent assay (ELISA) measurements were also performed to highlight both the 1000-fold performance enhancement of SPR and the ability to measure both spiked and native concentrations that are not achievable with ELISA.

SPR and SPR Imaging: Recent Trends in Developing Nanodevices for Detection and Real-Time Monitoring of Biomolecular Events

In this paper we review the underlying principles of the surface plasmon resonance (SPR) technique, particularly emphasizing its advantages along with its limitations regarding the ability to discriminate between the specific binding response and the interfering effects from biological samples. While SPR sensors were developed almost three decades, SPR detection is not yet able to reduce the time-consuming steps of the analysis, and is hardly amenable for miniaturized, portable platforms required in point-of-care (POC) testing. Recent advances in near-field optics have emerged, resulting in the development of SPR imaging (SPRi) as a powerful optical, label-free monitoring tool for multiplexed detection and monitoring of biomolecular events. The microarrays design of the SPRi chips incorporating various metallic nanostructures make these optofluidic devices more suitable for diagnosis and near-patient testing than the traditional SPR sensors. The latest developments indicate SPRi detection as being the most promising surface plasmon-based technique fulfilling the demands for implementation in lab-on-a-chip (LOC) technologies.

On-Demand Formation of Supported Lipid Membrane Arrays by Trehalose-Assisted Vesicle Delivery for SPR Imaging

The fabrication of large-scale, solid-supported lipid bilayer (SLB) arrays has traditionally been an arduous and complex task, primarily due to the need to maintain SLBs within an aqueous environment. In this work, we demonstrate the use of trehalose vitrified phospholipid vesicles that facilitate on-demand generation of microarrays, allowing each element a unique composition, for the label-free and high-throughput analysis of biomolecular interactions by SPR imaging (SPRi). Small, unilamellar vesicles (SUVs) are suspended in trehalose, deposited in a spatially defined manner, with the trehalose vitrifying on either hydrophilic or hydrophobic SPR substrates. SLBs are subsequently spontaneously formed on-demand simply by in situ hydration of the array in the SPR instrument flow cell. The resulting SLBs exhibit high lateral mobility, characteristic of fluidic cellular lipid membranes, and preserve the biological function of embedded cell membrane receptors, as indicated by SPR affinity measurements. Independent fluorescence and SPR imaging studies show that the individual SLBs stay localized at the area of deposition, without any encapsulating matrix, confining coral, or boundaries. The introduced methodology allows individually addressable SLB arrays to be analyzed with excellent label-free sensitivity in a real-time, high-throughput manner. Various protein-ganglioside interactions have been selected as a model system to illustrate discrimination of strong and weak binding responses in SPRi sensorgrams. This methodology has been applied toward generating hybrid bilayer membranes on hydrophobic SPR substrates, demonstrating its versatility toward a range of surfaces and membrane geometries. The stability of the fabricated arrays, over medium to long storage periods, was evaluated and found to be good. The highly efficient and easily scalable nature of the method has the potential to be applied to a variety of label-free sensing platforms requiring lipid membranes for high-throughput analysis of their properties and constituents.

Direct Detection of α-1 Antitrypsin in Serum Samples using Surface Plasmon Resonance with a New Aptamer-Antibody Sandwich Assay

The challenges associated with performing surface plasmon resonance (SPR) based measurements in serum and other biofluids have continued to limit the applicability of this valuable sensing technology for sensitive bioaffinity measurements of proteins in clinically relevant samples. In this paper, a new sandwich assay is introduced for the quantitative SPR analysis of α-1 antitrypsin (AAT), which is a recognized biomarker for Alzheimer's disease. Detection was performed via the specific adsorption of AAT onto a gold chip surface modified with a DNA aptamer. The measurement dynamic range and also sensitivity in serum were improved with the subsequent surface binding of antiAAT. A methodology was established to measure the target protein in serum, albumin and immunoglobulin G (IgG) solutions with the results correlated with measurements in buffer only. A comparison between SPR and enzyme-linked immunosorbent assay (ELISA) measurements was also made. The detection of AAT in serum at clinically relevant concentrations was demonstrated with target concentrations as low as 10 fM readily achievable.

Polydopamine-functionalization of graphene oxide to enable dual signal amplification for sensitive surface plasmon resonance imaging detection of biomarker

Surface plasmon resonance imaging (SPRi) is one of the powerful tools for immunoassays with advantages of label-free, real-time, and high-throughput; however, it often suffers from limited sensitivity. Herein we report a dual signal amplification strategy utilizing polydopamine (PDA) functionalization of reduced graphene oxide (PDA-rGO) nanosheets for sensitive SPRi immunoassay in serum. The PDA-rGO nanosheet is synthesized by oxidative polymerization of dopamine in a gentle alkaline solution in the presence of graphene oxide (GO) sheets and then is antibody-conjugated via a spontaneous reaction between the protein and the PDA component. In the dual amplification mode, the first signal comes from capture of the antibody-conjugated PDA-rGO to form sandwiched immunocomplexes on the SPRi chip, followed by a PDA-induced spontaneous gold reductive deposition on PDA-rGO to further enhance the SPRi signal. The detection limit as low as 500 pg mL(-1) is achieved on a nonfouling SPRi chip with high specificity and a wide dynamic range for a model biomarker, carcinoembryonic antigen (CEA) in 10% human serum.

Cellphone camera imaging of a periodically patterned chip as a potential method for point-of-care diagnostics

In this study, we demonstrate that a disposable chip periodically patterned with suitable ligands, an ordinary cellphone camera, and a simple pattern recognition software, can potentially be used for quantitative diagnostics. A key factor in this demonstration is the design of a calibration grid around the chip that, through a contrast transfer process, enables reliable analysis of the images collected under variable ambient lighting conditions. After exposure to a dispersion of amine terminated silica beads used as analyte mimicking pathogens, an epoxy-terminated glass substrate microcontact printed with octadecyltrichlorosilane (250 μm periodicity) developed a characteristic pattern of beads which could be easily imaged with a cellphone camera of 3.2 MP pixels. A simple pattern recognition algorithm using fast Fourier transform produced a quantitative estimate of the analyte concentration present in the test solution. In this method importantly, neither the chip fabrication process nor the fill-factor of the periodic pattern need be perfect to arrive at a conclusive diagnosis. The method suggests a viable platform that may potentially find use in fault-tolerant and robust point-of-care diagnostic applications.

Microarray technology for major chemical contaminants analysis in food: current status and prospects

Chemical contaminants in food have caused serious health issues in both humans and animals. Microarray technology is an advanced technique suitable for the analysis of chemical contaminates. In particular, immuno-microarray approach is one of the most promising methods for chemical contaminants analysis. The use of microarrays for the analysis of chemical contaminants is the subject of this review. Fabrication strategies and detection methods for chemical contaminants are discussed in detail. Application to the analysis of mycotoxins, biotoxins, pesticide residues, and pharmaceutical residues is also described. Finally, future challenges and opportunities are discussed.

Etched glass microarrays with differential resonance for enhanced contrast and sensitivity of surface plasmon resonance imaging analysis

We report the fabrication and characterization of gold-coated etched glass array substrates for surface plasmon resonance imaging (SPRi) analysis with significantly enhanced performance, in particular image contrast and sensitivity. The etching of the glass substrate induces a variation in the resonance condition and thus in the resonance angle between the etched wells and the surrounding area, leading to the isolation of the array spot resonance with a significant reduction of the background signal. FDTD simulations show arrays with large spots and minimal spot-to-spot spacing yield ideal differential resonance conditions, which are verified by experimental results. Simulations also indicate the etched well structure exhibits enhanced SPR electric field intensity by 3-fold as compared to standard planar gold chips. Changes in the bulk sensitivity of the etched arrays have been obtained at the 10(-4) RIU level based on image intensity difference. The strong image contrast allows for improved microarray imaging analysis with easily distinguished signals from background resonance. The etched array chips are demonstrated for SPRi detection of bacterial toxins through the coating of an ultrathin SiO(2) film for direct vesicle fusion that establishes a supported membrane-based biosensing interface. Protein detection with cholera toxin (CT) at 5 nM is obtained, making this chip one of the most sensitive SPR imaging substrates ever reported without a postbinding amplification scheme. Furthermore, the surface can be regenerated by Triton X-100 for repeated cycles of membrane formation, protein binding, and biomolecular removal. The reusability and enhanced performance of the etched glass array chips should find a broad range of applications, opening up new avenues for high-throughput SPR imaging detection with convenience and marked surface sensitivity.