Nanoscale color printing has recently emerged as a unique alternative to traditional pigments by providing record spatial resolution, angular independent, durable and single material colors. Widely based on plasmonic nanostructures, numerous efforts in the field have aimed at extending color range and saturation relying on a variety of designs and metals. Alternatively, silicon nanostructures support finely tunable electric and magnetic multipolar resonances, afford low absorption losses and benefit from well-established industrial fabrication processes, all features ideally suited to nanoscale color printing. Here we compare the properties of silicon nanodiscs with those of aluminum and silver plasmonic elements for the specific purpose of nanoscale color reproduction targeting the coverage of a broad and vivid color palette. We highlight the different properties of such metallic and dielectric resonators in various geometric and illumination conditions leading to the optimization of silicon nanodisc arrays for the fabrication of high resolution color features as well as millimetric paining replicas. The fabricated structures span a large, continuous color range with varying hue and saturation that is visible by conventional optical microscopy, photography as well as the bare eye under white light illumination. High-throughput electron beam lithography as well as color mixing schemes are discussed to further harness the unique properties of silicon nanodiscs as color elements, paving the way for a broader exploitation of nanoscale color printing.

The one-pot seedless protocol provides a facile approach in the synthesis of gold nanostars (AuNS) that involves only three reagents, gold (III) chloride (HAuCl 4 ), silver nitrate (AgNO 3 ) and ascorbic acid (C 6 H 8 O 6 ). While studies correlating the synthesis parameters of the seedmediated protocol to surface-enhance Raman scattering (SERS) enhancement is well reported, the same understanding of the one-pot seedless protocol is limited. Here, we aim to elucidate how the synthesis parameters of AuNS from the one-pot seedless protocol, the AuNS concentration, surface passivation and aggregation level affect the colloidal SERS enhancement. Using crystal violet (CV) as a Raman probe molecule, we found that the SERS enhancement increases with Au 3+ /C 6 H 8 O 6 molar ratio up to 0.60 and Au 3+ /Ag + molar ratio up to 18. Although the surfactant, cetyltrimethylammonium bromide (CTAB) maintained colloidal stability, it reduced the SERS enhancement. Interestingly, the SERS enhancement did not increase monotonically with AuNS concentration, but decreased when AuNS concentration was beyond 15 pM. The SERS enhancement also increased with the increasing level of salt-induced aggregation of AuNS, but only within a few minutes. While the concept of SERS with colloidal nanostructures is not new, we have shown for the first time, a detailed systematic study of various parameters that affect the SERS enhancement of AuNS synthesized using a one-pot seedless protocol. This study enables us to optimize the SERS enhancement of AuNS at the synthesis level to make them effective colloid-based SERS substrates for potential use in intracellular biosensing.

Enterovirus 71 (EV71) is a major public health threat that requires rapid point-of-care detection. Here, we developed a surface-enhanced Raman spectroscopy (SERS)-based scheme that utilized protein-induced aggregation of colloidal gold nanostars (AuNS) to rapidly detect EV71 without the need for fabricating a solid substrate, Raman labels or complicated sample handling. We used AuNS (hydrodynamic diameter, D of 105.12 ± 1.13 nm) conjugated to recombinant scavenger receptor class B, member 2 (SCARB2) protein with known affinity to EV71. In the absence of EV71, AuNS-SCARB2 aggregated in biological media and produced four enhanced Raman peaks at 390, 510, 670, and 910 cm. In the presence of EV71, the three peaks at 510, 670, and 910 cm disappeared, while the peak at 390 cm diminished in intensity as the virus bound to AuNS-SCARB2 and prevented them from aggregation. These three peaks (510, 670, and 910 cm) were potential markers for specific detection of EV71 as their disappearance was not observable with a different dengue virus (DENV) as our control. Furthermore, the Raman measurements from colloidal SERS were more sensitive in probing the aggregation of AuNS-SCARB2 for detecting the presence of EV71 in protein-rich samples compared to UV-vis spectrum measurements. With this facile "anti-aggregation" approach, we were able to detect EV71 in protein-rich biological medium within 15 min with reasonable sensitivity of 10 pfu/mL and minimal sample preparation, making this translatable for point-of-care applications.

The emergence and rapid spread of antibiotic resistance poses a serious threat to healthcare systems across the globe. The existence of carbapenemase-producing Enterobacteriaceae (CPE) such as Klebsiella pneumoniae renders the use of carbapenems, the last-resort class of β-lactam antibiotics, ineffective against bacterial infections, often leading to CPE-associated mortalities. Current methods of detection such as the Carba NP test and modified Hodge's test require hours to days to detect, which delays the response to isolate patients for rapid intervention. Here, we developed a surface-enhanced Raman scattering (SERS)-based detection scheme which utilizes gold nanostars conjugated to a β-lactam antibiotic ceftriaxone (CRO) as a beacon for rapid detection of bacterial β-lactamase secreted by Delhi metalloproteinase (NDM)-producing Escherichia coli as our CPE model with carbapenemase activity. The cleavage of β-lactam ring in CRO by NDM (Class B β-lactamase) caused a detectable reduction in SERS intensities at 722, 1358, and 1495 cm −1 within 25 min. Ratiometric analysis of the SERS peaks at 722, 1358, and 1495 cm −1 normalized against the Raman peak of polystyrene cuvette at 620 cm −1 showed the peak at 1358 cm −1 having the most significant change in intensity upon CPE detection. This reduced detection time has not been reported to date for CPE detection, and our novel approach using SERS could be extended to detect the activity of other classes of β-lactamases to broaden its clinical utility.