There is a need for surveillance of COVID-19 to identify individuals infected with SARS-CoV-2 coronavirus. Although specific, nucleic acid testing has limitations in terms of point-of-care testing. One potential alternative is the nonstructural protease (nsp5, also known as M pro /3CL pro ) implicated in SARS-CoV-2 viral replication but not incorporated into virions. Here, we report a divalent substrate with a novel design, (Cys) 2 –(AA) x –(Asp) 3 , to interface gold colloids in the specific presence of M pro leading to a rapid and colorimetric readout. Citrate- and tris(2-carboxyethyl)phosphine (TCEP)-AuNPs were identified as the best reporter out of the 17 ligated nanoparticles. Furthermore, we empirically determined the effects of varying cysteine valence and biological media on the sensor specificity and sensitivity. The divalent peptide was specific to M pro , that is, there was no response when tested with other proteins or enzymes. Furthermore, the M pro detection limits in Tris buffer and exhaled breath matrices are 12.2 and 18.9 nM, respectively, which are comparable to other reported methods (i.e., at low nanomolar concentrations) yet with a rapid and visual readout. These results from our work would provide informative rationales to design a practical and noninvasive alternative for COVID-19 diagnostic testing—the presence of viral proteases in biofluids is validated.

Plasmonic coupling via nanoparticle assembly is a popular signal-generation method in bioanalytical sensors. Here, we customized an all-peptide-based ligand that carries an anchoring group, polyproline spacer, biomolecular recognition, and zwitterionic domains for functionalizing gold nanoparticles (AuNPs) as a colorimetric enzyme sensor. Our results underscore the importance of the polyproline module, which enables the SARS-CoV-2 main protease (Mpro) to recognize the peptidic ligand on nanosurfaces for subsequent plasmonic coupling via Coulombic interactions. AuNP aggregation is favored by the lowered surface potential due to enzymatic unveiling of the zwitterionic module. Therefore, this system provides a naked-eye measure for Mpro. No proteolysis occurs on AuNPs modified with a control ligand lacking a spacer domain. Overall, this all-peptide-based ligand does not require complex molecular conjugations and hence offers a simple and promising route for plasmonic sensing other proteases.

Aromatic interactions are commonly involved in the assembly of naturally occurring building blocks, and these interactions can be replicated in an artificial setting to produce functional materials. Here we describe a colorimetric biosensor using co‐assembly experiments with plasmonic gold and surfactant‐like peptides (SLPs) spanning a wide range of aromatic residues, polar stretches, and interfacial affinities. The SLPs programmed in DDD−(ZZ)x−FFPC self‐assemble into higher‐order structures in response to a protease and subsequently modulate the colloidal dispersity of gold leading to a colorimetric readout. Results show the strong aggregation propensity of the FFPC tail without polar DDD head. The SLPs were specific to the target protease, i.e., Mpro, a biomarker for SARS‐CoV‐2. This system is a simple and visual tool that senses Mpro in phosphate buffer, exhaled breath condensate, and saliva with detection limits of 15.7, 20.8, and 26.1 nM, respectively. These results may have value in designing other protease testing methods.

Electrostatic interactions are a key driving force that mediates colloidal assembly. The Schulze-Hardy rule describes that counterions of high charge valence are easier for nanoparticle coagulation; this relationship has been...

Skin consists of a lamellar structure with diverse cell types (e.g., immune cells, melanocytes, and basal cells) that periodically detach from the basement membrane, move to the surface, and die for self-renewal. [3] Melanocytes are a critical cell type that generate melanin to absorb UV light (290-400 nm), which is a major risk for skin diseases (e.g., melanoma) due to DNA damage. [4][5][6][7] Here, melanin-containing organelles called melanosomes are transferred to the surrounding keratinocytes. This increase in melanosome concentration leads to darker skin phototypes, and darker phototypes can be a function of racial background or previous sun exposure, that is, tanning. [8] Indeed, skin pigmentation depends on variations in the size, number, clustering phase, and the proportions between melanin species (e.g., eumelanin and pheomelanin). [9] Skin pigmentation has been quantified using melanosome volume fraction (M f ) parameter: 1.3-6.3% for lightly pigmented adults, 11-16% for moderately pigmented adults, and 18-43% for darkly pigmented adults. [10] Variations in skin phototypes can complicate biomedical optics. Melanin absorption increases linearly from 800 to 600 nm and exponentially from 600 to 300 nm. [11,12] Darker skin phototypes can absorb and scatter more photons: as a result, incident light is attenuated before it reaches the target of interest, and signal transmission can be impeded back to the sensor. Therefore, variations in skin phototypes have negatively affected many forms of medical optic technology including pulse oximetry, [13,14] cerebral tissue oximeters, [15] optical coherence tomography, [16] wearable electronics, [17][18][19] photoacoustic (PA) imaging, [20] fluorescence imaging, [21] and photothermal therapy. [22] One recent study compared 48 097 pairs of oxygen saturation levels measured by pulse oximetry and arterial blood gas test obtained from 8675 white patients and 1326 black patients. [13] The results found that pulse oximetry had trouble in diagnosing hypoxemia in 11% Black patients and 3% white patients due to light absorption by melanin. [13,14] Furthermore, wearable electronics (e.g., smartwatches) have reported inaccuracies in heart rate readings occurring more often in users with dark skin than light skin. [17,18] Clearly, the impact of differences in skin phototypes underscore the ongoing need to understand and correct racial bias in optical technologies. While larger 3D-bioprinted skin-mimicking phantoms with skin colors ranging across the Fitzpatrick scale are reported. These tools can help understand the impact of skin phototypes on biomedical optics. Synthetic melanin nanoparticles of different sizes (70-500 nm) and clusters are fabricated to mimic the optical behavior of melanosome. The absorption coefficient and reduced scattering coefficient of the phantoms are comparable to real human skin. Further the melanin content and distribution in the phantoms versus real human skins are validated via photoacoustic (PA) imaging. The PA signal of the phantom can be improved ...