Nanoparticles (NPs) will inevitably interact with proteins and form protein coronas once they are exposed to biological fluids.

Noble metal nanoclusters (NCs) show great promise as nanoprobes for bioanalysis and cellular imaging in biological applications due to ultrasmall size, good photophysical properties, and excellent biocompatibility. In order to achieve a comprehensive understanding of possible biological implications, a series of spectroscopic measurements were conducted under different temperatures to investigate the interactions of Au NCs (∼1.7 nm) with three model plasmatic proteins (human serum albumin (HSA), γ-globulins, and transferrin). It was found that the fluorescence quenching of HSA and γ-globulins triggered by Au NCs was due to dynamic quenching mechanism, while the fluorescence quenching of transferrin by Au NCs was a result of the formation of a Au NC-transferrin complex. The apparent association constants of the Au NCs bound to HSA, γ-globulins, and transferrin demonstrated no obvious difference. Thermodynamic studies demonstrated that the interaction between Au NCs and HSA (or γ-globulins) was driven by hydrophobic forces, while the electrostatic interactions played predominant roles in the adsorption process for transferrin. Furthermore, it was proven that Au NCs had no obvious interference in the secondary structures of these three kinds of proteins. In turn, these three proteins had a minor effect on the fluorescence intensity of Au NCs, which made fluorescent Au NCs promising in biological applications owing to their chemical and photophysical stability. In addition, by comparing the interactions of small molecules, Au NCs, and large nanomaterials with serum albumin, it was found that the binding constants were gradually increased with the increase of particle size. This work has elucidated the interaction mechanisms between nanoclusters and proteins, and shed light on a new interaction mode different from the protein corona on the surface of nanoparticles, which will highly contribute to the better design and applications of fluorescent nanoclusters.

Enzymes are an important class of biomacromolecules which catalyze many metabolic processes in living systems. Nanomaterials can be synthesized with tailored sizes as well as desired surface modifications, thus acting as promising enzyme regulators. Fluorescent gold nanoclusters (AuNCs) are a representative class of ultrasmall nanoparticles (USNPs) with sizes of ∼2 nm, smaller than most of proteins including enzymes. In this work, we chose α-chymotrypsin (ChT) and AuNCs as the model system. Activity assays and inhibition kinetics studies showed that dihydrolipoic acid (DHLA)-coated AuNCs (DHLA-AuNCs) had a high inhibitory potency (IC 50 = 3.4 μM) and high inhibitory efficacy (>80%) on ChT activity through noncompetitive inhibition mechanism. In distinct contrast, glutathione (GSH)-coated AuNCs (GSH-AuNCs) had no significant inhibition effects. Fluorescence spectroscopy, agarose gel electrophoresis and circular dichroism (CD) spectroscopy were conducted to explore the underlying mechanisms. A two-step interaction model was proposed. First, both DHLA-AuNCs and GSH-AuNCs might be bound to the positively charged sites of ChT through electrostatic forces. Second, further hydrophobic interactions occurred between three tyrosine residues of ChT and the hydrophobic carbon chain of DHLA, leading to a significant structural change thus to deactivate ChT on the allosteric site. On the contrary, no such interactions occurred with GSH of zwitterionic characteristic, which explained no inhibitory effect of GSH-AuNCs on ChT. To the best of our knowledge, this is the first example of the allosteric inhibition of ChT by nano regulators. These findings provide a fundamental basis for the design and development of nano regulators.

The oral absorption of exenatide, a drug for type 2 diabetes treatment, can be improved by using nanoparticles (NPs) for its delivery. To improve the mucus penetration and intestinal absorption of exenatide, we designed a block copolymer, CSKSSDYQC-dextran-poly­(lactic-co-glycolic acid) (CSK-DEX-PLGA), and used it for the preparation of exenatide-loaded NPs. The functionalized exenatide-loaded NPs composed of CSK-DEX-PLGA were able to target intestinal epithelial cells and reduce the mucus-blocking effect of the intestine. Moreover, the CSK modification of DEX-PLGA was found to significantly promote the absorption efficiency of NPs in the small intestine based on in vitro ligation of the intestinal rings and an examination of different intestinal absorption sites. Compared to DEX-PLGA-NPs (DPs), the absorption of CSK-DEX-PLGA-NPs (CDPs) was increased in the villi, allowing the drug to act on gobletlike Caco-2 cells through clathrin-, caveolin-, and gap-mediated endocytosis. Furthermore, the enhanced transport ability of CDPs was observed in a study on Caco-2/HT-29-MTX cocultured cells. CDPs exhibited a prolonged hypoglycemic response with a relative bioavailability of 9.2% in diabetic rats after oral administration. In conclusion, CDPs can target small intestinal goblet cells and have a beneficial effect on the oral administration of macromolecular peptides as a nanometer-sized carrier.

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Background The use of drug nanocarriers to encapsulate drugs for oral administration may become an important strategy in addressing the challenging oral absorption of some drugs. In this study—with the premise of controlling single variables—we prepared model nanoparticles with different particle sizes, surface charges, and surface hydrophobicity/hydrophilicity. The two key stages of intestinal nanoparticles (NPs) absorption—the intestinal mucus layer penetration stage and the trans-intestinal epithelial cell stage—were decoupled and analyzed. The intestinal absorption of each group of model NPs was then investigated. Results Differences in the behavioral trends of NPs in each stage of intestinal absorption were found to result from differences in particle properties. Small size, low-magnitude negative charge, and moderate hydrophilicity helped NPs pass through the small intestinal mucus layer more easily. Once through the mucus layer, an appropriate size, positive surface charge, and hydrophobic properties helped NPs complete the process of transintestinal epithelial cell transport. Conclusions To achieve high drug bioavailability, the basic properties of the delivery system must be suitable for overcoming the physiological barrier of the gastrointestinal tract.

Noble metal nanoclusters (NCs) have been widely used in bioimaging and bioanalysis due to their unique molecular-like structures and good biocompatibility. Bright nanomaterials with high quantum yields are in need for widespread applications. Unfortunately, the weak photoluminescence (PL) of metal NCs hampers their biomedical applications, and thus it is urgent to develop effective routes to enhance their brightness, especially in aqueous solutions. In this work, we reported a facile strategy to prepare highly luminescent Au x Ag1–x nanocomposites (x: molar ratio of Au) by electrostatic-induced assembly of nonluminescent glutathione (GSH) stabilized silver NCs (GSH-Ag NCs) and weak orange-emitting GSH stabilized gold NCs (GSH-Au NCs) in aqueous solutions. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), fluorescence spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS), UV–vis absorption spectroscopy, and dynamic light scattering (DLS) shed light on the mechanism of PL enhancement. It was found that the positively charged gold nanoclusters and the negatively charged silver nanoclusters formed aggregates by electrostatic force, leading to a 40-fold fluorescence intensity enhancement compared with GSH-Au NCs. This was a novel method to strengthen the fluorescence of nanoclusters with such large enhancement in aqueous solutions. With the molar ratio of Au and Ag changing from 80:1 to 2:3, the emission maximum of the Au x Ag1–x nanocomposites could be tuned from 590 to 548 nm. The electrostatic force of the Au0.50Ag0.50 nanocomposites enabled them to respond to pH. The Au0.50Ag0.50 nanocomposites were fluorescent turn-on and turn-off at pH 2.6 and pH 7.5, respectively. In this respect, they can be used as a fluorescent switch and be further used as a general recyclable pH probe in the range of 2.6–7.5. This work will inspire even better strategies to further improve the brightness of noble metal NCs.

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