Tetrahydrocannabinol (THC) is the main active component in marijuana and the rapid detection of THC in human body fluid plays a critical role in forensic analysis and public health. Surface-enhanced Raman scattering (SERS) sensing has been increasingly used to detect illicit drugs; however, only limited SERS sensing results of THC in methanol solution have been reported, while its presence in body fluids, such as saliva or plasma, has yet to be investigated. In this article, we demonstrate the trace detection of THC in human plasma and saliva solution using a SERS-active substrate formed by in situ growth of silver nanoparticles (Ag NPs) on diatom frustules. THC at extremely low concentration of 1 pM in plasma and purified saliva solutions were adequately distinguished with good reproducibility. The SERS peak at 1603 cm–1 with standard deviation of 3.4 cm–1 was used for the evaluation of THC concentration in a methanol solution. Our SERS measurement also shows that this signature peak experiences a noticeable wavenumber shift and a slightly wider variation in the plasma and saliva solution. Additionally, we observed that THC in plasma or saliva samples produces a strong SERS peak at 1621 cm–1 due to the stretching mode of OCO, which is related to the metabolic change of THC structures in body fluid. To conduct a quantitative analysis, principal component analysis (PCA) was applied to analyze the SERS spectra of 1 pM THC in methanol solution, plasma, and purified saliva samples. The maximum variability of the first three principal components was achieved at 71%, which clearly denotes the impact of different biological background signals. Similarly, the SERS spectra of THC in raw saliva solution under various metabolic times were studied using PCA and 98% of the variability is accounted for in the first three principal components. The clear separation of samples measured at different THC resident times can provide time-dependent information on the THC metabolic process in body fluids. A linear regression model was used to estimate the metabolic rate of THC in raw saliva and the predicted metabolic time in the testing data set matched well with the training data set. In summary, the hybrid plasmonic-biosilica SERS substrate can achieve ultrasensitive, near-quantitative detection of trace levels of THC in complex body fluids, which can potentially transform forensic sensing techniques to detect marijuana abuse.

Optofluidic devices are particularly well-suited for biological and chemical sensing. Especially, nanophotonic sensors employed in optofluidics have greatly overcome the limitations of conventional optical sensors in terms of size, sensitivity, specificity, tunability, photostability, and in vivo applicability. [1] Equally important, microfluidic devices enable facile delivery of sample solution to the sensing region and allow for high throughput detection. However, the limit of mass transport imposed by the laminar flow inside the microfluidic channel dictates the sensitivity and throughput of optofluidic devices. [2] In past years, various optofluidic devices have been developed using microring resonators, [3] metamaterials, [4] surface plasmon resonance (SPR), [5] and surfaceenhanced Raman scattering (SERS). [6] As a vibrational spectroscopic technique, SERS can provide specific information about the molecular structure by the strong local optical field enhancement generated by plasmonic nanoparticles (NPs) or nanostructures. [7] Most published optofluidic-SERS results were obtained utilizing either 1) colloidal metallic NPs flowing inside microfluidic channels, or 2) SERS-active plasmonic nanostructures fabricated on the surface of the microfluidic channels to provide the necessary SERS enhancement factors (EFs). Plasmonic NPs, primarily based on silver (Ag) or gold (Au), such as colloidal metallic NPs, [8] nanorods, [9] nanostars, [10] core-shell NPs, [11] and Surface-enhanced Raman scattering (SERS) sensing in microfluidic devices, namely optofluidic-SERS, suffers an intrinsic tradeoff between mass transport and hot spot density, both of which are required for ultrasensitive detection. To overcome this compromise, photonic crystal-enhanced plasmonic mesocapsules are synthesized, utilizing diatom biosilica decorated with in-situ growth silver nanoparticles (Ag NPs). In the optofluidic-SERS testing of this study, 100× higher enhancement factors and more than 1,000× better detection limit are achieved compared with traditional colloidal Ag NPs, the improvement of which is attributed to unique properties of the mesocapsules. First, the porous diatom biosilica frustules serve as carrier capsules for high density Ag NPs that form high density plasmonic hot-spots. Second, the submicron-pores embedded in the frustule walls not only create a large surface-to-volume ratio allowing for effective analyte capture, but also enhance the local optical field through the photonic crystal effect. Last, the mesocapsules provide effective mixing with analytes as they are flowing inside the microfluidic channel. The reported mesocapsules achieve single molecule detection of Rhodamine 6G in microfluidic devices and are further utilized to detect 1 × 10 −9 m of benzene and chlorobenzene compounds in tap water with near real-time response, which successfully overcomes the constraint of traditional optofluidic sensing.

When myocardial walls experience stress due to cardiovascular diseases, like heart failure, hormone N-terminal pro-B-type natriuretic peptide (NT-proBNP) is secreted into the blood. Early detection of NT-proBNP can assist diagnosis of heart failure and enable early medical intervention. A simple, cost-effective detection technique such as the widely used fluorescence imaging immunoassay is yet to be developed to detect clinically relevant levels of NT-proBNP. In this work, we demonstrate photonic crystal-enhanced fluorescence imaging immunoassay using diatom biosilica, which is capable of detecting low levels of NT-proBNP in solution with the concentration range of 0~100 pg/mL. By analyzing the fluorescence images in the spatial and spatial frequency domain with principle component analysis (PCA) and partial least squares regression (PLSR) algorithms, we create a predictive model that achieves great linearity with a validation R 2 value of 0.86 and a predictive root mean square error of 14.47, allowing for good analyte quantification. To demonstrate the potential of the fluorescence immunoassay biosensor for clinical usage, we conducted qualitative screening of high and low concentrations of NT-proBNP in human plasma. A more advanced machine learning algorithm, the support vector machine classification, was paired with the PCA and trained by 160 fluorescence images. In the 40 testing images, we achieved excellent specificity of 93%, as well as decent accuracy and sensitivity of 78% and 65% respectively. Therefore, the photonic crystal-enhanced fluorescence *

Diatoms are single‐celled algae that biologically fabricate nanostructured silica shells with ordered pore arrays called frustules that resemble a 2D photonic crystal. A monolayer of Pinnularia frustules isolated from cell culture is deposited on a glass substrate and then conformally coated with silver nanoparticles (AgNPs) to serve as a nanostructured thin film for ultrathin layer chromatography (UTLC). Malachite green and Nile red are resolved in toluene mobile phase and the separated analytes are profiled micro‐Raman spectroscopy, where plasmonic AgNPs provide surface‐enhanced Raman scattering (SERS). The AgNP‐diatom frustule monolayer improves SERS detection of malachite green by an average factor of 1.8 ± 0.1 over the plasmonic AgNP layer on glass. Analysis of hot spots on the AgNP‐diatom frustule monolayer reveals that nearly 20% of the SERS active area intensifies the SERS signal at least tenfold over the SERS signal for AgNP on glass. Diatom‐SERS enhancement is attributed to guided‐mode resonances of the Raman laser source, which in turn further enhances the localized surface plasmonic resonance from AgNPs. Overall, the AgNP‐diatom frustule monolayer thin film is a new functional material that uniquely enables separation of analytes by UTLC, quantitative SERS detection of separated analytes, and photonic enhancement of the SERS signals.

Surface-enhanced Raman scattering (SERS) has started to attract attention in vapor sensing; however, practical applications require shorter response time and better sensitivity. Herein, we report a facile multiscale SERS substrate for trace-level detection of vapors using a portable Raman spectrometer through the synergistic integration of biologically fabricated diatom photonic crystals and gold−silica core−shell nanoparticles. The multiscale substrate is composed of (1) a micrometer-scaled, 3dimensional, diatom biosilica frustule enabling efficient vapor− substrate interaction for rapid sensing, (2) periodic pores, on the order of 100 nm, inducing plasmonic-photonic coupled resonances for enhanced SERS signals, (3) gold nanoparticle cores, with a diameter on the order of 10 nm, contributing plasmonic field enhancements, and (4) porous 1 nm thick silica core−shells enabling analyte vapor adsorption and concentration. The combination of the hierarchal, multiscale features results in a SERS substrate capable of rapid and sensitive detection of target vapors in air. The multiscale substrate's functionality is characterized using the polycyclic aromatic hydrocarbon pyrene, and the contribution from each scale is verified by using a stagnant vapor chamber. The sensor equilibrates in only 3 min, and detection is achieved down to 1 ppm. The sensor is then applied to the detection of explosive 2,4-dinitrotoluene vapor below 100 ppb in an airflow chamber to replicate practical detection conditions, achieving detection in under 3 min at room temperature and under 1 min when heated. This work successfully demonstrates detection of explosive vapor and represents a significant advancement toward widespread vapor sensing via SERS.