Modulation of Gold Nanorod Growth via the Proteolysis of Dithiol Peptides for Enzymatic Biomarker Detection

Colorimetric sensing for translational applications: from colorants to mechanisms

Colorimetric sensing offers instant reporting via visible signals. Versus labor-intensive and instrument-dependent detection methods, colorimetric sensors present advantages including short acquisition time, high throughput screening, low cost, portability, and a user-friendly approach. These advantages have driven substantial growth in colorimetric sensors, particularly in point-of-care (POC) diagnostics. Rapid progress in nanotechnology, materials science, microfluidics technology, biomarker discovery, digital technology, and signal pattern analysis has led to a variety of colorimetric reagents and detection mechanisms, which are fundamental to advance colorimetric sensing applications. This review first summarizes the basic components (e.g., color reagents, recognition interactions, and sampling procedures) in the design of a colorimetric sensing system. It then presents the rationale design and typical examples of POC devices, e.g., lateral flow devices, microfluidic paper-based analytical devices, and wearable sensing devices. Two highlighted colorimetric formats are discussed: combinational and activatable systems based on the sensor-array and lock-and-key mechanisms, respectively. Case discussions in colorimetric assays are organized by the analyte identities. Finally, the review presents challenges and perspectives for the design and development of colorimetric detection schemes as well as applications. The goal of this review is to provide a foundational resource for developing colorimetric systems and underscoring the colorants and mechanisms that facilitate the continuing evolution of POC sensors.

Protease detection in the biosensor era: A review

Proteases have been proposed as potential biomarkers for several pathological conditions including cancers, multiple sclerosis and cardiovascular diseases, due to their ability to break down the components of extracellular matrix and basement membrane. The development of protease biosensors opened up the possibility to investigate the proteolytic activity of dysregulated proteases with higher efficiency over the traditional detection assays due to their quick detection capability, high sensitivity and selectivity, simple instrumentation and cost-effective fabrication processes. In contrast to the recently published review papers that primarily focused on one specific class of proteases or one specific detection method, this review article presents different optical and electrochemical detection methods that can be used to design biosensors for all major protease families. The benefits and drawbacks of various transducer techniques integrated into protease biosensing platforms are analyzed and compared. The main focus is on activity-based biosensors that use peptides as biorecognition elements. The effects of nanomaterials on biosensor performance are also discussed. This review should help readers to select the biosensor that best fits their needs, and contribute to the further development of this research field. Protease biosensors may allow better comprehension of protease overexperession and potentially enable novel devices for point-of-care testing.

Empirical Optimization of Peptide Sequence and Nanoparticle Colloidal Stability: The Impact of Surface Ligands and Implications for Colorimetric Sensing

Surface ligands play a critical role in controlling and defining the properties of colloidal nanocrystals. These aspects have been exploited to design nanoparticle aggregation-based colorimetric sensors. Here, we coated 13-nm gold nanoparticles (AuNPs) with a large library of ligands (e.g., from labile monodentate monomers to multicoordinating macromolecules) and evaluated their aggregation propensity in the presence of three peptides containing charged, thiolate, or aromatic amino acids. Our results show that AuNPs coated with the polyphenols and sulfonated phosphine ligands were good choices for electrostatic-based aggregation. AuNPs capped with citrate and labile-binding polymers worked well for dithiol-bridging and π-π stacking-induced aggregation. In the example of electrostatic-based assays, we stress that good sensing performance requires aggregating peptides of low charge valence paired with charged NPs with weak stability and vice versa. We then present a modular peptide containing versatile aggregating residues to agglomerate a variety of ligated AuNPs for colorimetric detection of the coronavirus main protease. Enzymatic cleavage liberates the peptide segment, which in turn triggers NP agglomeration and thus rapid color changes in <10 min. The protease detection limit is 2.5 nM.

Peptide Amphiphile Mediated Co-assembly for Nanoplasmonic Sensing

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.

Spacer Matters: All-Peptide-Based Ligand for Promoting Interfacial Proteolysis and Plasmonic Coupling

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 (M^pro) 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 M^pro. 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.

Gold-Silver Core-Shell Nanoparticle Crosslinking Mediated by Protease Activity for Colorimetric Enzyme Detection

Plasmonic nanoparticles produce a localized surface plasmon resonance (LSPR) under optical excitation. The LSPR of nanoparticles can shift in response to changes in the local dielectric environment and produce a color change. This color change can be observed by the naked eye due to the exceptionally large extinction coefficients (10⁸-10¹¹ M^-1 cm^-1) of plasmonic nanoparticles. Herein, we investigate the optical shifts (i.e., color change) of three unique gold-silver core-shell nanoparticle structures in response to changes in their dielectric environment upon nanoparticle aggregation. Aggregation is induced by a cysteine-containing peptide that has a sulfhydryl near its N and C termini, which crosslinks nanoparticles. Furthermore, we demonstrate that adding proline spacers between the cysteines impacts the degree of aggregation and, ultimately, the color response. Using this information, we construct a colorimetric enzyme assay, where the signal produced from nanoparticle aggregation is modulated by proteolysis. The degree of aggregation and the resulting optical shift can be correlated with enzyme concentration with high linearity (R² = 0.998). Overall, this study explores the optical properties of gold-silver core-shell nanoparticles in a dispersed vs aggregated state and leverages that information to develop an enzyme sensor with a spectral LOD of 0.47 ± 0.09 nM.

One-Step Supramolecular Multifunctional Coating on Plant Virus Nanoparticles for Bioimaging and Therapeutic Applications

Plant viral nanoparticles (plant VNPs) are promising biogenetic nanosystems for the delivery of therapeutic, immunotherapeutic, and diagnostic agents. The production of plant VNPs is simple and highly scalable through molecular farming in plants. Some of the important advances in VNP nanotechnology include genetic modification, disassembly/reassembly, and bioconjugation. Although effective, these methods often involve complex and time-consuming multi-step protocols. Here, we report a simple and versatile supramolecular coating strategy for designing functional plant VNPs via metal-phenolic networks (MPNs). Specifically, this method gives plant viruses [e.g., tobacco mosaic virus (TMV), cowpea mosaic virus, and potato virus X] additional functionalities including photothermal transduction, photoacoustic imaging, and fluorescent labeling via different components in MPN coating [i.e., complexes of tannic acid (TA), metal ions (e.g., Fe3+, Zr4+, or Gd3+), or fluorescent dyes (e.g., rhodamine 6G and thiazole orange)]. For example, using TMV as a viral substrate by choosing Zr4+-TA and rhodamine 6G, fluorescence is observed peaking at 555 nm; by choosing Fe3+-TA coating, the photothermal conversion efficiency was increased from 0.8 to 33.2%, and the photoacoustic performance was significantly improved with a limit of detection of 17.7 μg mL-1. We further confirmed that TMV@Fe3+-TA nanohybrids show good cytocompatibility and excellent cell-killing performance in photothermal therapy with 808 nm irradiation. These findings not only prove the practical benefits of this supramolecular coating for designing multifunctional and biocompatible plant VNPs but also bode well for using such materials in a variety of plant virus-based theranostic applications.

A Charge-Switchable Zwitterionic Peptide for Rapid Detection of SARS-CoV-2 Main Protease

The transmission of SARS-CoV-2 coronavirus has led to the COVID-19 pandemic. Nucleic acid testing while specific has limitations for mass surveillance. One alternative is the main protease (Mpro ) due to its functional importance in mediating the viral life cycle. Here, we describe a combination of modular substrate and gold colloids to detect Mpro via visual readout. The strategy involves zwitterionic peptide that carries opposite charges at the C-/N-terminus to exploit the specific recognition by Mpro . Autolytic cleavage releases a positively charged moiety that assembles the nanoparticles with rapid color changes (t<10 min). We determine a limit of detection for Mpro in breath condensate matrices <10 nM. We further assayed ten COVID-negative subjects and found no false-positive result. In the light of simplicity, our test for viral protease is not limited to an equipped laboratory, but also is amenable to integrating as portable point-of-care devices including those on face-coverings.

Peptidic Sulfhydryl for Interfacing Nanocrystals and Subsequent Sensing of SARS-CoV-2 Protease

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 Mpro/3CLpro) 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 Mpro 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 Mpro, that is, there was no response when tested with other proteins or enzymes. Furthermore, the Mpro 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.

Acidity-Activated Charge Conversion of 177Lu-Labeled Nanoagent for the Enhanced Photodynamic Radionuclide Therapy of Cancer

Nanomaterials in combination with radionuclide therapy (RNT) provide new opportunities for cancer treatment. However, nanomaterials with efficient tumor accumulation have been less exploited for effective radionuclide-based therapy. Here, we report glycol chitosan-based nanoparticles (GCP-NPs) with acidic pH-dependent surface charge conversion for efficient radionuclide-based combination therapy. The nanoplatform can change the surface charge of nanoparticles from slight negative to positive in the acidic tumor microenvironment, which facilitates cellular internalization and penetration and thus improves the tumor accumulation efficiency of nanomaterials. Radiolabeling of GCP-NPs with ^99mTc enables in vivo radioactive imaging in the mouse subcutaneous tumor model, showing 8.1-fold enhanced tumor uptake relative to pH-insensitive control nanoparticles (termed as GCOP-NPs). Afterward, therapeutic radioisotope ¹⁷⁷Lu-labeled GCP-NPs (¹⁷⁷Lu-GCP-NPs) that utilize RNT synergistic with photodynamic therapy (PDT) derived from conjugated pyropheophorbide-a within nanoparticles endow superior antitumor efficacy in living cells and tumor-bearing mouse model. More importantly, the combination of RNT and PDT using ¹⁷⁷Lu-GCP-NPs can effectively inhibit lung metastasis and eliminate splenomegaly, which is not possible for individual RNT or PDT. Therefore, this study proposes a facile radionuclide-based combination therapy strategy toward complete cancer remission.