Figure 3.

M^pro-induced color change using the modular peptide amphiphiles and colloidal gold (citrate). (a) The time progression of optical absorption of AuNPs (3.4 nM, 100 μL) when incubated with D₃F₂C₁ intact (left) and its fragments (right, c = 1.5 μM). The curves were recorded every 1 min for 30 min. Arrows designate sizable optical changes at 520 and 600 nm. (b-c) The concentration- and time-dependent color evolution of AuNPs (3.4 nM, 100 μL) in the presence of intact D₃F₂C₁ and its pre-cleaved fragments. These are cropped images with a color bar where purple represents particle flocculation. See also TEM images of AuNPs (3.4 nM, 8 μL) when mixed with D₃F₂C₁ intact (d, monolayer) and its fragments (e, heterogeneous stacking). (f) DLS profiles of AuNPs (3.4 nM, 100 μL) incubated with D₃F₂C₁ intact (blue) and its fragments (red) of 0 – 100 μM. (g-h) View of MANTA^[22b] size measurement shows that AuNPs (c = 0.2 – 0.6 nM) scatter blue light with D₃F₂C₁ intact (2.0 μM), whereas the fragment-induced colloidal aggregates scatter red light. (i) Zeta potential of AuNPs (3.4 nM, 100 μL) when incubated with increasing concentrations of D₃F₂C₁ intact (blue, reduced from −26.0 to −40.4 mV) and its fragments (red, increased from −26.6 to −9.0 mV). Error bars represent triplicate measurements for one sample. (j) The agarose gel (0.7% w/v) electrophoresis image collected from citrate-AuNPs only, AuNPs incubated with HS-PEG_2k-OCH₃, and intact D₃F₂C₁ (from left to right). Samples were prepared using the AuNPs (~15 nM, 40 μL) mixed with glycerol (10 μL). Note that TBE buffer (1×) promotes instant aggregation of citrate-AuNPs. (k) Ca²⁺ cation (20 mM)-modulated dispersity of citrate (red), PEGylated (green), and D₃F₂C₁-capped AuNPs (blue). The colloidal dispersity is quantified by ratiometric signal, i.e., Abs₆₀₀/Abs₅₂₀. The DDD stretch negatively charges the surfaces and promotes colloidal stability via electrostatic double repulsion. (l-n) White-light image (top) and quantified reversal color change (bottom) of the gold pellet in different surfactant solutions (10 mM, 100 μL) or solvents (100 μL). Panel l indicates an aromatic stacking-driven co-assembly of D₃F₂C₁ fragments and AuNPs. The gold pellet is prepared by aggregating AuNPs (3.4 nM, 100 μL) with D₃F₂C₁/D₃Y₂C₁/D₃F₁C₁ fragments (at 600 μM, 30 μL). Error bars = standard deviations (n = 3).