Investigation of DNA Hybridization on Nano-Structured Plasmonic Surfaces for Identifying Nasopharyngeal Viruses.

Recently, studies have revealed that human herpesvirus 4 (HHV-4), also known as the Epstein-Barr virus, might be associated with the severity of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Compared to SARS-CoV-2 infection alone, patients coinfected with SARS-CoV-2 and HHV-4 had higher risks of fever, inflammation, and even death, thus, confirming that HHV-4/SARS-CoV-2 coinfection in patients could benefit from clinical investigation. Although several intelligent devices can simultaneously discern multiple genes related to SARS-CoV-2, most operate via label-based detection, which restricts them from directly measuring the product. In this study, we developed a device that can replicate and detect SARS-CoV-2 and HHV-4 DNA. This device can conduct a duplex polymerase chain reaction (PCR) in a microfluidic channel and detect replicates in a non-labeled manner through a plasmonic-based sensor. Compared to traditional instruments, this device can reduce the required PCR time by 55% while yielding a similar amount of amplicon. Moreover, our device's limit of detection (LOD) reached 100 fg/mL, while prior non-labeled sensors for SARS-CoV-2 detection were in the range of ng/mL to pg/mL. Furthermore, the device can detect desired genes by extracting cells artificially infected with HHV-4/SARS-CoV-2. We expect that this device will be able to help verify HHV-4/SARS-CoV-2 coinfected patients and assist in the evaluation of practical treatment approaches.

Biomimetic affinity sensor for the ultrasensitive detection of neonicotinoids.

Multiple pesticides are often used in combination to protect crops from pests. This makes rapid on-site detection of pesticide contamination challenging. Herein, we describe a method for simultaneous detection of diverse neonicotinoid pesticides using a sensor that combines neonicotinoid-specific odorant-binding protein 2 (OBP2), which was cloned from an insect chemical sensing protein and modified gold nanoparticles with local surface plasmon resonance (LSPR)-based digital nanoplasmonometry (DiNM). When neonicotinoid pesticides bind to OBP2 on gold nanoparticles, the induced LSPR shift peak wavelength is too small to be measured using conventional LSPR immunoassays. DiNM records and compares the scattered image intensity in two adjacent wavelength bands, A and B, centered on the LSPR peak. It considers both the peak shift and the relative intensity change in these two bands, resulting in a significant LSPR signal enhancement. Then the spectral-image contrast was computed as the signal response. Using this approach, we obtained excellent limits of detection (LODs) of 1.4, 1.5, and 4.5 ppb for the neonicotinoids imidacloprid, acetamiprid, and dinotefuran, respectively. Blind tests demonstrated high positive and negative rates for teas, approximately 85 and 100%, respectively. Recombinant OBP2 produced in E. coli offers several advantages over antibodies, including high yield, time savings, and cost effectiveness. Moreover, this method is highly selective and sensitive to neonicotinoids, making it practical for field use.

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.

Hybrid modes in gold nanoslit arrays on Bragg nanostructures and their application for sensitive biosensors.

In this work, we present high-performance surface plasmonic sensors using gold nanostructures and Bragg photonic structures. The gold film on the Bragg structure provides Tamm plasmon states (TPs). The Fano coupling between higher order TPs and Bloch-wave surface plasmon polariton (BW-SPP) on the gold nanoslit array results in a new hybrid Tamm-plasmon mode. Using finite-difference time-domain calculations, we demonstrate that the hybrid mode has the advantages of high surface sensitivity of BW-SPP mode and high resonant quality of Tamm state. The calculated plasmonic field distribution shows that the hybrid mode has a similar evanescent distribution with BW-SPP mode on gold surface and TPs field in the Bragg structure. The experimental results verify that the hybrid mode has one hundred times higher wavelength sensitivity than the Tamm state. The figure of merit of the hybrid mode is five times better than the BW-SPP mode in conventional nanoslit arrays. The real-time sensorgram further confirms that the hybrid mode has a much higher sensitivity and better signal to noise ratios in the biomolecular interaction measurement.

Dual Gold-Nanoslit Electrodes for Ultrasensitive Detection of Antigen-Antibody Reactions in Electrochemical Surface Plasmon Resonance.

We present the use of surface charges in dual gold-nanoslit electrodes to improve the surface plasmon resonance (SPR) detection limit by several orders of magnitude. The SPR is directly generated by gold-nanoslit arrays in the two electrodes. The SPR shifts for both nanoslit arrays are measured simultaneously with a simple hyperspectral setup. When biomolecules are captured by specific antibodies on the dual electrodes, the surface charge is changed during the electrochemical process due to the increase in surface impedance. The push-pull-type electrodes generate opposite surface charges. Using the differences in both spectral shifts, the change in surface charge is detected sensitively. We demonstrate that using a [Fe(CN)6]3-/4- redox process after antigen-antibody interactions, the dual nanoslit electrodes show an enhancement of the detection limit from 1 µg/mL to 10 pg/mL.

A Co-Printed Nanoslit Surface Plasmon Resonance Structure in Microfluidic Device for LMP-1 Detection.

This paper reports a novel micro/nanostructure co-hot embossing technique. Gold-capped nanostructures were used as localized surface plasmon resonance (SPR) sensors and were integrated into a microfluidic channel. The advantage of the co-hot embossing technique is that the SPR sensors do not need to be aligned with the microfluidic channel while bonding to it. The integrated SPR sensor and microfluidic channel were first characterized, and the sensitivity of the SPR sensor to the refractive index was found using different concentrations of glycerol solutions. The SPR sensor was also used to quantify latent membrane protein (LMP-1) when modifying anti-LMP-1 at the surface of the SPR sensor. Different concentrations of LMP-1 samples were used to build a calibration curve.

Sensitive Oligonucleotide Detection Using Resonant Coupling between Fano Resonance and Image Dipoles of Gold Nanoparticles.

The surface plasmon resonance (SPR)-based sensor has been widely used for biodetection. One of the attractive roles is the gold nanostructure with Fano resonance. Its sharp resonant profile takes advantage of the high figure of merit (FoM) in high-sensitivity detection. However, it is still difficult to detect small molecules at low concentrations due to the extremely low refractive index changes on the metallic surface. We propose using the coupling of image dipoles of gold nanoparticles (AuNPs) and Fano resonance of periodic capped gold nanoslits (CGNs) for sensitive small-molecule detections. The coupling mechanism was verified by three-dimensional finite-difference time-domain calculations and experiments. AuNPs on CGN form image dimer assemblies and induce image dipole with resonance wavelengths ranging from 730 to 550 nm. The surface plasmon polaritons (SPPs) interact with the image dipole of the AuNP on the CGNs and then scatter out through the periodic gold caps. The experimental results show that the peak intensity of grating resonance is decreased by the effect of image dipole and exhibits the maximum intensity change when the Fano resonance matches the resonance of image dipole. The 50 nm AuNPs can be detected with a surface density of less than one particle/µm2 by using the intensity change as the signal. With the resonant coupling between Fano resonance and image dipole extinction, the oligonucleotide with a molecular weight of 5.5 kDa can be detected at a concentration of 100 fM. The resonant coupling dramatically pushes the sensitivity boundary, and we report the limit of detection (LOD) to be 3 orders of magnitude lower than that of the prism-based SPR. This study provides a promising and efficient method for detecting low concentrations of small molecules such as aptamers, miRNA, mRNA, and peptides.

Spectral image contrast-based flow digital nanoplasmon-metry for ultrasensitive antibody detection.

BACKGROUND: Gold nanoparticles (AuNPs) have been widely used in local surface plasmon resonance (LSPR) immunoassays for biomolecule sensing, which is primarily based on two conventional methods: absorption spectra analysis and colorimetry. The low figure of merit (FoM) of the LSPR and high-concentration AuNP requirement restrict their limit of detection (LOD), which is approximately ng to µg mL-1 in antibody detection if there is no other signal or analyte amplification. Improvements in sensitivity have been slow in recent for a long time, and pushing the boundary of the current LOD is a great challenge of current LSPR immunoassays in biosensing. RESULTS: In this work, we developed spectral image contrast-based flow digital nanoplasmon-metry (Flow DiNM) to push the LOD boundary. Comparing the scattering image brightness of AuNPs in two neighboring wavelength bands near the LSPR peak, the peak shift signal is strongly amplified and quickly detected. Introducing digital analysis, the Flow DiNM provides an ultrahigh signal-to-noise ratio and has a lower sample volume requirement. Compared to the conventional analog LSPR immunoassay, Flow DiNM for anti-BSA detection in pure samples has an LOD as low as 1 pg mL-1 within only a 15-min detection time and 500 µL sample volume. Antibody assays against spike proteins of SARS-CoV-2 in artificial saliva that contained various proteins were also conducted to validate the detection of Flow DiNM in complicated samples. Flow DiNM shows significant discrimination in detection with an LOD of 10 pg mL-1 and a broad dynamic detection range of five orders of magnitude. CONCLUSION: Together with the quick readout time and simple operation, this work clearly demonstrated the high sensitivity and selectivity of the developed Flow DiNM in rapid antibody detection. Spectral image contrast and digital analysis further provide a new generation of LSPR immunoassay with AuNPs.

Continuous polymerase chain reaction microfluidics integrated with a gold-capped nanoslit sensing chip for Epstein-Barr virus detection.

We present the first combination of a microfluidic polymerase chain reaction (PCR) with a gold nanoslit-based surface plasmon resonance (SPR) sensor for detecting the DNA sequence of latent membrane protein 1 (LMP1). The PCR microchannel was produced through a laser scribing technique, and the SPR nanoslit chip was manufactured via hot-embossing nanoimprinting lithography. Afterward, the LMP1 DNA probe was adsorbed onto the SPR chip of the integrated device through electrostatic interactions for further detection. The device can complete the analytical procedure in around 36 min, while the traditional machine requires 105 min to achieve similar signals under the same PCR thermal cycles. The calibration curve with serially diluted LMP1 DNA exhibited the accuracy (R2 > 0.99) and sensitivity (limit of detection: ∼10-11 g/mL) of the device. Moreover, extracted DNA from Epstein-Barr virus (EBV)-positive cells were directly detected through the integrated chip. In brief, this all-in-one chip can amplify gene fragments at the front-end and detect them at the back-end, decreasing the time required for the analysis without compromising accuracy or sensitivity. We believe this label-free, real-time, low-cost device has enormous potential for rapid detection of various viruses, such as EBV and COVID-19.

Polyglutamine-Specific Gold Nanoparticle Complex Alleviates Mutant Huntingtin-Induced Toxicity.

Huntington's disease (HD) belongs to protein misfolding disorders associated with polyglutamine (polyQ)-rich mutant huntingtin (mHtt) protein inclusions. Currently, it is indicated that the aggregation of polyQ-rich mHtt participates in neuronal toxicity and dysfunction. Here, we designed and synthesized a polyglutamine-specific gold nanoparticle (AuNP) complex, which specifically targeted mHtt and alleviated its toxicity. The polyglutamine-specific AuNPs were prepared by decorating the surface of AuNPs with an amphiphilic peptide (JLD1) consisting of both polyglutamine-binding sequences and negatively charged sequences. By applying the polyQ aggregation model system, we demonstrated that AuNPs-JLD1 dissociated the fibrillary aggregates from the polyQ peptide and reduced its ß-sheet content in a concentration-dependent manner. By further integrating polyethyleneimine (PEI) onto AuNPs-JLD1, we generated a complex (AuNPs-JLD1-PEI). We showed that this complex could penetrate cells, bind to cytosolic mHtt proteins, dissociate mHtt inclusions, reduce mHtt oligomers, and ameliorate mHtt-induced toxicity. AuNPs-JLD1-PEI was also able to be transported to the brain and improved the functional deterioration in the HD Drosophila larva model. Our results revealed the feasibility of combining AuNPs, JLD1s, and cell-penetrating polymers against mHtt protein aggregation and oligomerization, which hinted on the early therapeutic strategies against HD.

Nanoplasmonic Structure of a Polycarbonate Substrate Integrated with Parallel Microchannels for Label-Free Multiplex Detection.

In this study, a multiplex detection system was proposed by integrating a localized surface plasmon resonance (LSPR) sensing array and parallel microfluidic channels. The LSPR sensing array was fabricated by nanoimprinting and gold sputter on a polycarbonate (PC) substrate. The polydimethylsiloxane (PDMS) microfluidic channels and PC LSPR sensing array were bound together through (3-aminopropyl)triethoxysilane (APTES) surface treatment and oxygen plasma treatment. The resonant spectrum of the LSPR sensing device was obtained by broadband white-light illumination and polarized wavelength measurements with a spectrometer. The sensitivity of the LSPR sensing device was measured using various ratios of glycerol to water solutions with different refractive indices. Multiplex detection was demonstrated using human immunoglobulin G (IgG), IgA, and IgM. The anti-IgG, anti-IgA, and anti-IgM were separately modified in each sensing region. Various concentrations of human IgG, IgA, and IgM were prepared to prove the concept that the parallel sensing device can be used to detect different targets.

Self-referencing biosensors using Fano resonance in periodic aluminium nanostructures.

Surface plasmon resonance (SPR) is an important technique for real-time and label-free detection of specific binding biomolecules. However, conventional SPR signals come from both the surface binding biomolecules and the variation in the bulk refractive index. This work demonstrates that Fano resonance in an aluminum capped nanoslit array has the ability to remove the signal of bulk refractive index changes from the SPR signal. As compared to gold nanostructures, the aluminum nanostructure provides an asymmetrical Fano resonance with clear peak and dip wavelengths. The peak wavelength is close to the grating resonance condition. The evanescent depth at the peak wavelength is up to several microns. The dip wavelength comes from the SPR effect. The evanescent depth at the dip wavelength is about 300 nm. By simultaneously measuring the shifts of peaks and the dip wavelengths, the variation in the bulk refractive index can be removed and only the biolayer thickness is measured. The finite-difference time-domain calculation shows that the 470 nm-period nanoslit array with 90 and 70 nm slit depths has the optimal thickness sensitivity. In this experiment, a simple multispectral imaging system is developed for multiple bio-interaction measurements. The measured results verify that the bulk refractive index changes can be removed and the surface biomolecular interactions can be directly obtained without the need of a reference channel.

Dispersion-enhancing surface treatment of AuNPs for a reduced probe loading and detection limit using t-SPR detection.

COVID-19 has shown that a highly specific and rapid diagnostic system is a necessity. A spectral imaging-based surface plasmon resonance (SPRi) platform with an integrated microfluidic biosensor to detect oligonucleotide sequences has been proposed to be a promising alternative for infectious diseases due to its safe and straightforward use. Approaches to reduce the DNA probe loading onto gold nanoparticles with various types of polyethylene glycol (PEG) were explored. Here, we demonstrated the stability of functionalised gold nanoparticles with unmodified PEG whilst lowering the probe loading density. The system was evaluated by performing the detection of a mimicking COVID-19 target sequence, single point-mutation sequence and fully mismatch sequence. Highly specific binding of the mimicking COVID-19 target sequence was observed and analysed by the spectral imaging SPR approach. Our work has demonstrated the potential of a controlled probe density using unmodified PEG as an especially promising functionalisation strategy in SPR spectral imaging assays.

Enhancing Raman signals from bacteria using dielectrophoretic force between conductive lensed fiber and black silicon.

An osmium-coated lensed fiber (OLF) probe combined with a silver-coated black silicon (SBS) substrate was used to generate a dielectrophoretic (DEP) force that traps bacteria and enables Raman signal detection from bacteria. The lensed fiber coated with a 2-nm osmium layer was used as an electrode for the DEP force and also as a lens to excite Raman signals. The black silicon coated with a 150-nm silver layer was used both as the surface-enhanced Raman scattering (SERS) substrate and the counter electrode. The enhanced Raman signal was collected by the same OLF probe and further analyzed with a spectrometer. For Raman measurements, a drop of bacterial suspension was placed between the OLF probe and the SBS substrate. By controlling the frequency of an AC voltage on the OLF probe and SBS substrate, a DEP force at 1 MHz concentrated bacteria on the SBS surface and removed the unbound micro-objects in the solution at 1 kHz. A bacteria concentration of 6 × 104 CFU/mL (colony forming units per mL) could be identified in less than 15 min, using a volume of only 1 µL, by recording the variation of the Raman peak at 740 cm-1.

Screening anti-metastasis drugs by cell adhesion-induced color change in a biochip.

Metastasis is a frequent complication of cancer and accounts for more than 60% of patients' mortality. Despite technological advancements, treatment options are still limited. Ion channels participate in the regulation of cell adhesion, whilst the regulation of cell adhesion further controls metastasis formation. However, to develop a new ion channel inhibitor targeting metastasis takes tremendous effort and resources; therefore, drug repurposing is an emerging strategy in oncology. In previous studies, we have developed a metal-based nanoslit surface plasmon resonance (SPR) platform to examine the influence of drugs on the cell adhesion process. In this work, we developed a scanner-based cell adhesion kinetic examination (CAKE) system that is capable of monitoring the cell adhesion process by measuring color changes of SPR biosensors. The system's performance was demonstrated by screening the anti-metastasis ability of compounds from a commercial ion-channel inhibitor library. Out of the 274 compounds from the inhibitor library, zinc pyrithione (ZPT) and terfenadine were demonstrated to influence CL1-5 cell adhesion. The cell responses to the two compounds were then compared with those by traditional cell adhesion assays where similar behavior was observed. Further investigation of the two compounds using wound healing and transwell assays was performed and inhibitions of both cell migration and invasion by the two compounds were also observed. The results indicate that ZPT and terfenadine are potential candidates for anti-metastasis drugs. Our work has demonstrated the label-free drug screening ability of our CAKE system for finding potential drugs for cancer treatment.

Large shift of resonance wavelengths of silver nanoslit arrays using electrowetting-on-dielectric cells.

A simple design for shifting the resonance wavelength of silver nanoslits using an electrowetting-on-dielectric (EWOD) cell is proposed. The EWOD cell comprises a polycarbonate (PC) substrate with Teflon-coated silver nanoslits and a glass substrate with Teflon-coated electrodes. A glycerol droplet is placed between the two substrates, and out of the path of a probe beam at zero electric field. Application of an electric field smaller than 0.3 V/µm on the electrodes moves the glycerol droplet into the path of the probe beam, shifting the resonance wavelength of the silver nanoslits by 135 nm. A change (0.33) in the refractive index of the effective medium that is adjacent to the silver nanoslits causes a large shift in the resonance wavelength. The spectral shift of the silver nanoslits is repeatable by the electric field. This simple design is a great achievement for high-performance electro-optical devices with large wavelength shift ranges such as optical switches, variable optical attenuators, and sensor applications.

Combination of Capped Gold Nanoslit Array and Electrochemistry for Sensitive Aqueous Mercuric Ions Detection.

Label-free surface plasmon resonance (SPR) detection of mercuric ions in various aqueous solutions, using capped gold nanoslit arrays combined with electrochemical (EC) sensing technique, is demonstrated. The nanoslit arrays are fabricated on flexible cyclo-olefin polymer substrates by a nanoimprinting lithography method. The EC and SPR signals for the investigation of current responses and transmission SPR spectra are simultaneously measured during metal ions electrodeposition. Glycerol-water solution is studied to evaluate the resonant peak wavelength sensitivity (480.3 nm RIU-1) with a FOM of 40.0 RIU-1 and the obtained intensity sensitivity is 1819.9%. The ferrocyanide/ferricyanide redox couple performs the diffusion controlled electrochemical processes (R2 = 0.99). By investigating the SPR intensity changes and wavelength shifts of various mercuric ion concentrations, the optical properties are evaluated under chronoamperometric conditions. The sensors are evaluated in the detection range between 100 µM and 10 nM with a detection limit of 1 µM. The time dependence of SPR signals and the selectivity of 10 µM Hg2+ in the presence of 10 µM interfering metal ion species from Ca2+, Co2+, Ni2+, Na+, Cu2+, Pb2 + and Mn2+ are determined. The capped gold nanoslit arrays show the selectivity of Hg2+ and the EC sensing method is effectively utilized to aqueous Hg2+ detection. This study provides a label-free detection technique of mercuric ions and this developed system is potentially applicable to detecting chemicals and biomolecules.

Multichannel nanoplasmonic platform for imidacloprid and fipronil residues rapid screen detection.

In recent years, imidacloprid and fipronil have been reported to harm beneficial insects, such as honey bees, and to potentially pose risks to mammals, including humans. Considering their widespread use and potential minimum toxic range from 10 ppb to 1 ppm (species dependent), a simple, rapid, sensitive, and reliable method for screening and detection is urgently needed. Here, we present a surface plasmon resonance (SPR)-based nanoplasmonic chip integrated with a multichannel spectral imaging system to detect ecosystem-harming pesticides. The pre-modification of the designed mercapto-haptens reduced detection time to 2.5 h. Moreover, owing to the multichannel configuration, it was possible to introduce an internal standard analytical method to effectively reduce matrix interference in real samples; thus, the concentration of the target pesticide could be determined more precisely. The strong linearity of the spiked sample test results indicated high accuracy in quantifying target pesticides. Considering the limit of detection (~10 ppb), the cutoffs for detection and quantification were set at 15 and 45 ppb, respectively, and were used as the detection criteria. The detection results of the blind tests of real samples were also compared with those of liquid chromatography electrospray ionization tandem mass spectrometry (standard method) and were highly consistent. The custom-made integrated SPR system allows much simpler, label-free, high-throughput, and reliable on-site identification and quantification of imidacloprid and fipronil. All test results validated the platform's capability in the on-site rapid screening and detection of pesticide residues at the parts per billion and parts per million levels.

High-Throughput and Dynamic Study of Drug and Cell Interactions Using Contrast Images in Aluminum-Based Nanoslit Arrays.

High-throughput and dynamic measurement for living cell activities can benefit biological research and drug development. A low-cost metallic nanostructure-based surface plasmon resonance (SPR) imaging platform, comprising multiple aluminum nanoslit arrays and a color image device, is proposed for label-free study of cell and drug interactions. The multiple nanoslit sensing arrays were fabricated using the compression-injection molding process. These sensing chips showed a detectable depth of 600 nm and refractive index resolution of ∼5 × 10-5 refractive index unit (RIU) by using a self-referenced two-color analysis. Two examples of kinetic studies of living cells under various doses of drugs are presented. The focal adhesion kinases inhibitor (FAKi 14) and cell interactions show exponential changes of cellular adhesion and time constants for different concentrations of antiadhesion drugs. The anticancer drug (doxorubicin (DOX))-treated cells show slow increases of SPR signals in the first 2 h due to the nucleus swelling. The DOX-treated cells further process plasma membrane disruption and become floating cells and debris in the medium, resulting in rapid drops of the SPR signals.

Combination of an Axicon Fiber Tip and a Camera Device into a Sensitive Refractive Index Sensor.

An axicon fiber tip combined with a camera device is developed to sensitively detect refractive indexes in solutions. The transparent axicon tips were made by etching optical fibers through a wet end-etching method at room temperature. When the axicon fiber tip was immersed in various refractive index media, the angular spectrum of the emitted light from the axicon fiber tip was changed. Using a low numerical aperture lens to collect the directly transmitted light, a high intensity sensitivity was achieved when the tip cone angle was about 35 to 40 degrees. We combined the axicon fiber tip with a laser diode and a smartphone into a portable refractometer. The front camera of the smartphone was used to collect the light emitted from the axicon fiber tip. By analyzing the selected area of the captured images, the refractive index can be distinguished for various solutions. The refractive index sensitivity was up to 56,000%/RIU, and the detection limit was 1.79 × 10-5 RIU. By measuring the refractive index change via the axicon fiber tip, the concentration of different mediums can be sensitively detected. The detection limits of the measurement for sucrose solutions, saline solutions, and diluted wine were 8.86 × 10-3 °Bx, 0.12‱, and 0.35%, respectively.