To tackle the COVID-19 outbreak, which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), there is an unmet need for highly accurate diagnostic tests at all stages of infection with rapid results and high specificity. Here, we present a label-free nanoplasmonic biosensor-based, multiplex screening test for COVID-19 that can quantitatively detect 10 different biomarkers (6 viral nucleic acid genes, 2 spike protein subunits, and 2 antibodies) with a limit of detection in the aM range, all within one biosensor platform. Our newly developed nanoplasmonic biosensors demonstrate high specificity, which is of the upmost importance to avoid false responses. As a proof of concept, we show that our detection approach has the potential to quantify both IgG and IgM antibodies directly from COVID-19-positive patient plasma samples in a single instrument run, demonstrating the high-throughput capability of our detection approach. Most importantly, our assay provides receiving operating characteristics, areas under the curve of 0.997 and 0.999 for IgG and IgM, respectively. The calculated p -value determined through the Mann–Whitney nonparametric test is <0.0001 for both antibodies when the test of COVID-19-positive patients ( n = 80) is compared with that of healthy individuals ( n = 72). Additionally, the screening test provides a calculated sensitivity (true positive rate) of 100% (80/80), a specificity (true negative rate) >96% (77/80), a positive predictive value of 98% at 5% prevalence, and a negative predictive value of 100% at 5% prevalence. We believe that our very sensitive, multiplex, high-throughput testing approach has potential applications in COVID-19 diagnostics, particularly in determining virus progression and infection severity for clinicians for an appropriate treatment, and will also prove to be a very effective diagnostic test when applied to diseases beyond the COVID-19 pandemic.

As interest continues to grow in Ti3C2T x and other related MXenes, advancement in methods of manipulation of their surface functional groups beyond synthesis-based surface terminations (T x : −F, −OH, and O) can provide mechanisms to enhance solution processability as well as produce improved solid-state device architectures and coatings. Here, we report a chemically important surface modification approach in which “solvent-like” polymers, polyethylene glycol carboxylic acid (PEG6-COOH), are covalently attached onto MXenes via esterification chemistry. Surface modification of Ti3C2T x with PEG6-COOH with large ligand loading (up to 14% by mass) greatly enhances dispersibility in a wide range of nonpolar organic solvents (e.g., 2.88 mg/mL in chloroform) without oxidation of Ti3C2T x two-dimensional flakes or changes in the structure ordering. Furthermore, cooperative interactions between polymer chains improve the nanoscale assembly of uniform microstructures of stacked MXene-PEG6 flakes into ordered thin films with excellent electrical conductivity (∼16,200 S·cm–1). Most importantly, our covalent surface modification approach with ω-functionalized PEG6 ligands (ω-PEG6-COOH, where ω: −NH2, −N3, −CHCH2) allows for control over the degree of functionalization (incorporation of valency) of MXene. We believe that installing valency onto MXenes through short, ion conducting PEG ligands without compromising MXenes’ features such as solution processability, structural stability, and electrical conductivity further enhance MXenes surface chemistry tunability and performance and widens their applications.

Surface-enhanced Raman scattering (SERS) is an ultrasensitive analytical technique, which is capable of providing high specificity, thus it can be used for toxicological drug assay (detection and quantification). However, SERS-based drug analysis directly in human biofluids requires mitigation of fouling and non-specificity effects that are commonly appeared from unwanted adsorption of endogenous biomolecules present in biofluids (e.g., blood plasma and serum) onto the SERS substrate. Here we report a bottom-up fabrication strategy to prepare ultrasensitive SERS substrates, firstly by functionalizing chemically synthesized gold triangular nanoprisms (Au TNPs) with poly(ethylene glycol)-thiolate in solid-state to avoid protein fouling, and secondly by generating flexible plasmonic patches to enhance SERS sensitivity via the formation of high intensity electromagnetic hot spots. Poly(ethylene glycol)-thiolate-functionalized Au TNPs in the form of flexible plasmonic patches show two-fold improved signal-to-noise ratio in comparison to triethylamine-passivated Au TNPs. Furthermore, the plasmonic patches display a SERS enhancement factor of 4.5 x 10 7 . Utilizing the Langmuir adsorption model we determine the adsorption constant of drugs for two different surface ligands and observed that the drug molecules display stronger affinity for poly(ethylene glycol) ligands than triethylamine. Our density functional theory calculations unequivocally support the interaction between drug molecules and poly(ethylene glycol) moieties. Furthermore, the universality of the plasmonic patch for SERSbased drug detection is demonstrated for cocaine, JWH-018, and opioids (fentanyl, despropionyl fentanyl, and heroin) and binary mixture (trace amount of fentanyl in heroin) analysis. We demonstrate that applicability of flexible plasmonic patches for the selective assay of fentanyl at picogram/milliliter concentration levels from drug-of-abuse patients' blood plasma. The fentanyl concentration determined in the patients' blood plasma from SERS analysis is in excellent agreement with the values determined by paper spray ionization mass spectrometry technique. We believe that the flexible plasmonic patch fabrication strategy would be widely applicable to any plasmonic nanostructures for SERS-based chemical sensing for clinical toxicology and therapeutic drug monitoring.

It is becoming understood that microRNAs hold great promise for noninvasive liquid biopsies for screening for different types of cancer, but current state-of-the-art RT-PCR and microarray techniques have sensitivity limitations that currently restrict their use. Herein, we report a new transduction mechanism involving delocalization of photoexcited conduction electrons wave function of gold triangular nanoprism (Au TNP) in the presence of -ssDNA/microRNA duplexes. This plasmoelectronic effect increases the electronic dimension of Au TNPs and substantially affects their localized surface plasmon resonance (LSPR) properties that together allow us to achieve a sensitivity for microRNA assay as low as 140 zeptomolar concentrations for our nanoplasmonic sensors. We show that the position of a single base-pair mismatch in the -ssDNA/microRNA duplex dramatically alters the LSPR properties and detection sensitivity. The unprecedentedly high sensitivity of nanoplasmonic sensors has allowed us to assay four different microRNAs (microRNA-10b, -182, -143, and -145) from bladder cancer patient plasma (50 μL/sample). For the first time, we demonstrate the utility of a label-free, nanoplasmonic sensor in quantification of tumor suppressor microRNAs, the level of tumor suppressor microRNAs goes down in a cancer patient as compared to normal healthy individuals, in metastatic and nonmetastatic bladder cancer patient plasma. Our statistical analysis of patient samples unequivocally suggests that the tumor suppressor microRNAs are more specific biomarkers (p-value of <0.0001) than oncogenic microRNAs for differentiation between metastatic and nonmetastatic bladder cancer, and nonmetastatic cancer from healthy individuals. This work demonstrating the electron wave functions delocalization dependent ultrasensitive LSPR properties of noble metal nanoparticles has a great potential for fabrication of miniaturized and extremely powerful sensors to investigate microRNA properties in other cancers (for example breast, lung, and pancreatic) through liquid biopsy.

There is an unmet need in clinical point-of-care (POC) cancer diagnostics for early state disease detection, which would greatly increase patient survival rates. Currently available analytical techniques for early stage cancer diagnosis do not meet the requirements for POC of a clinical setting. They are unable to provide the high demand of multiplexing, high-throughput, and ultrasensitive detection of biomarkers directly from low volume patient samples (“liquid biopsy”). To overcome these current technological bottle-necks, herein we present, for the first time, a bottom-up fabrication strategy to develop plasmonic nanoantenna-based sensors that utilize the unique localized surface plasmon resonance (LSPR) properties of chemically synthesized gold nanostructures, gold triangular nanoprisms (Au TNPs), gold nanorods (Au NRs), and gold spherical nanoparticles (Au SNPs). Our Au TNPs, NRs, and SNPs display refractive index unit (RIU) sensitivities of 318, 225, and 135 nm/RIU respectively. Based on the RIU results, we developed plasmonic nanoantenna-based multiplexing and high-throughput biosensors for the ultrasensitive assay of microRNAs. MicroRNAs are directly linked with cancer development, progression, and metastasis, thus they hold promise as next generation biomarkers for cancer diagnosis and prognosis. The developed biosensors are capable of assaying five different types of microRNAs at an attomolar detection limit. These sets of microRNAs include both oncogenic and tumor suppressor microRNAs. To demonstrate the efficiency as a POC cancer diagnostic tool, we analyzed the plasma of 20-bladder cancer patients without any sample processing steps. Importantly, our liquid biopsy-based biosensing approach is capable of differentiating healthy from early (“non-metastatic”) and late (“metastatic”) stage cancer with a p value <0.0001. Further, receiver operating characteristic analysis shows that our biosensing approach is highly specific, with an area under the curve of 1.0. Additionally, our plasmonic nanoantenna-based biosensors are regenerative, allowing multiple measurements using the same biosensors, which is essential in low- and middle-income countries. Taken together, our multiplexing and high-throughput biosensors have the unmatched potential to advance POC diagnostics and meet global needs for early stage detection of cancer and other diseases (e.g., infectious, autoimmune, and neurogenerative diseases).