Di-Arginine Additives for Dissociation of Gold Nanoparticle Aggregates: A Matrix-Insensitive Approach with Applications in Protease Detection

We report the reversible aggregation of gold nanoparticles (AuNPs) assemblies via a di-arginine peptide additive and thiolated PEGs (HS-PEGs). The AuNPs were first aggregated by attractive forces between the citrate-capped surface and the arginine side chains. We found that the HS-PEG thiol group has a higher affinity for the AuNP surface, thus leading to redispersion and colloidal stability. In turn, there was a robust and obvious color change due to on/off plasmonic coupling. The assemblies' dissociation was directly related to the HS-PEG structural properties such as their size or charge. As an example, HS-PEGs with a molecular weight below 1 kDa could dissociate 100% of the assemblies and restore the exact optical properties of the initial AuNP suspension (prior to the assembly). Surprisingly, the dissociation capacity of HS-PEGs was not affected by the composition of the operating medium and could be performed in complex matrices such as plasma, saliva, bile, urine, cell lysates, or even seawater. The high affinity of thiols for the gold surface encompasses by far the one of endogenous molecules and is thus favored. Moreover, starting with AuNPs already aggregated ensured the absence of a background signal as the dissociation of the assemblies was far from spontaneous. Remarkably, it was possible to dry the AuNP assemblies and solubilize them back with HS-PEGs, improving the colorimetric signal generation. We used this system for protease sensing in biological fluids. Trypsin was chosen as the model enzyme, and highly positively charged peptides were conjugated to HS-PEG molecules as cleavage substrates. The increase of positive charge of the HS-PEG-peptide conjugate quenched the dissociation capacity of the HS-PEG molecules, which could only be restored by the proteolytic cleavage. Picomolar limit of detection was obtained as well as the detection in saliva or urine.

A Protease-Responsive Polymer/Peptide Conjugate and Reversible Assembly of Silver Clusters for the Detection of Porphyromonas gingivalis Enzymatic Activity

We report the reversible aggregation of silver nanoparticle (AgNP) assemblies using the combination of a cationic arginine-based peptide and sulfur-capped polyethylene glycol (PEG). The formation and dissociation of the aggregates were studied by optical methods and electron microscopy. The dissociation of silver clusters depends on the peptide sequence and PEG size. A molecular weight of 1 kDa for PEG was optimal for the dissociation. The most important feature of this dissociation method is that it can operate in complex biofluids such as plasma, saliva, bile, urine, cell media, or even seawater without a significant decrease in performance. Moreover, the peptide-particle assemblies are highly stable and do not degrade (or express of loss of signal upon dissociation) when dried and resolubilized, frozen and thawed, or left in daylight for a month. Importantly, the dissociation capacity of PEG can be reduced via the conjugation of a peptide-cleavable substrate. The dissociation capacity is restored in the presence of an enzyme. Based on these findings, we designed a PEG-peptide hybrid molecule specific to the Porphyromonas gingivalis protease RgpB. Our motivation was that this bacterium is a key pathogen in periodontitis, and RgpB activity has been correlated with chronic diseases including Alzheimer's disease. The RgpB limit of detection was 100 pM RgpB in vitro. This system was used to measure RgpB in gingival crevicular fluid (GCF) samples with a detection rate of 40% with 0% false negatives versus PCR for P. gingivalis (n = 37). The combination of PEG-peptide and nanoparticles dissociation method allows the development of convenient protease sensing that can operate independently of the media composition.

Activatable prodrug for controlled release of an antimicrobial peptide via the proteases overexpressed in Candida albicans and Porphyromonas gingivalis

Candida albicans and Porphyromonas gingivalis are prevalent in the subgingival area where the frequency of fungal colonization increases with periodontal disease. Candida's transition to a pathogenic state and its interaction with P. gingivalis exacerbate periodontal disease severity. However, current treatments for these infections differ, and combined therapy remains unexplored. This work is based on an antimicrobial peptide that is therapeutic and induces a color change in a nanoparticle reporter. Methods: We built and characterized two enzyme-activatable prodrugs to treat and detect C. albicans and P. gingivalis via the controlled release of the antimicrobial peptide. The zwitterionic prodrug quenches the antimicrobial peptide's activity until activation by a protease inherent to the pathogens (SAP9 for C. albicans and RgpB for P. gingivalis). The toxicity of the intact prodrugs was evaluated against fungal, bacterial, and mammalian cells. Therapeutic efficacy was assessed through microscopy, disk diffusion, and viability assays, comparing the prodrug to the antimicrobial peptide alone. Finally, we developed a colorimetric detection system based on the aggregation of plasmonic nanoparticles. Results: The intact prodrugs showed negligible toxicity to cells absent a protease trigger. The therapeutic impact of the prodrugs was comparable to that of the antimicrobial peptide alone, with a minimum inhibitory concentration of 3.1 - 16 µg/mL. The enzymatic detection system returned a detection limit of 10 nM with gold nanoparticles and 3 nM with silver nanoparticles. Conclusion: This approach offers a convenient and selective protease sensing and protease-induced treatment mechanism based on bioinspired antimicrobial peptides.

Activatable Photoacoustic Probe for Imaging Infection: Gold Nanorod Dissociation In Vivo Reports Bacterial Protease Activity

We present a strategy for constructing activatable photoacoustic imaging (PAI) probes for in vivo enzyme activity measurements, based on a dissociation strategy previously applied to in vitro sensing. Unlike conventional nanoparticle aggregation strategies, dissociation minimizes false positives and functions effectively in complex biological environments. Overcoming the challenge of dissociating nanostructure aggregates, which arises from the strong van der Waals forces at short distances, we demonstrate the controlled assembly and dissociation of citrate-capped gold nanorods (AuNRs-citrate) using a diarginine peptide additive and a thiolated polyethylene glycol (HS-PEG-OMe), respectively. This assembly dissociation mechanism enables precise control of the optical and photoacoustic (PA) properties of AuNRs in both in vitro and in vivo settings. Building on these findings, we engineered an enzyme-sensitive PAI probe (AuNRs@RgpB) composed of AuNR assemblies and a PEG-peptide conjugate with a protease-specific cleavage sequence. The probe detects Arg-specific gingipain (RgpB), a protease expressed by Porphyromonas gingivalis associated with periodontal disease and Alzheimer's disease. Proteolytic cleavage of the peptide sequence triggers AuNR dissociation, resulting in enhanced PA signal output. The probe was designed to be injected intrathecally for preclinical trials to image gingipains and investigate the value of gingipain inhibitors developed for Alzheimer's disease. The probe's performance was characterized in vitro using UV-vis spectroscopy and PAI, achieving detection limits of 5 and 20 nM, respectively. In vivo studies involved intracranial injection of AuNRs@RgpB into RgpB-containing murine models, with PA monitoring over time. RgpB activity produced a four-fold PA signal increase within 2 h, while P. gingivalis-infected mice showed similar signal enhancement. Specificity was confirmed by negligible responses to Fusobacterium nucleatum, a non-RgpB-producing bacterium. Additionally, the system demonstrated utility in drug development by successfully monitoring the inhibition of RgpB activity using RgpB inhibitors (leupeptin and KYT-1) in vivo models. Beyond its immediate application to RgpB detection, this modular approach to plasmonic-based sensing holds significant potential for detecting other proteases, advancing both nanotechnology and protease-targeted diagnostics.