"Grafting-To" Covalent Binding of Plasmonic Nanoparticles onto Silica WGM Microresonators: Mechanically Robust Single-Molecule Sensors and Determination of Activation Energies from Single-Particle Events

Abstract

We hereby present a novel "grafting-to"-like approach for the covalent attachment of plasmonic nanoparticles (PNPs) onto whispering gallery mode (WGM) silica microresonators. Mechanically stable optoplasmonic microresonators were employed for sensing single-particle and single-molecule interactions in real time, allowing for the differentiation between binding and non-binding events. An approximated value of the activation energy for the silanization reaction occurring during the "grafting-to" approach was obtained using the Arrhenius equation; the results agree with available values from both bulk experiments and ab initio calculations. The "grafting-to" method combined with the functionalization of the plasmonic nanoparticle with appropriate receptors, such as single-stranded DNA, provides a robust platform for probing specific single-molecule interactions under biologically relevant conditions.

Keywords: WGM microresonators; gold nanorods (GNr); plasmonic nanoparticles (PNPs); silanization reaction.

Figure 1

Sequential modification of citrate stabilized Au-NR using a two-step procedure. Step 1, Au-S chemisorption on the PNPs; Step 2, condensation reactions between silanol moieties present on the microresonator surface and MPTMS (3-mercaptopropyl-trimethoxysilane)-modified Au-NR (Au-NR@HS-Si(OH)_n).

Figure 2

(a) Zeta potential versus pH for silica microspheres in aqueous solutions and after activation with alkaline piranha solution. (b,c) Time series for the variation of wavelength position (Δλ) and linewidth (Δκ) obtained in experiments with the WGM microresonators exposed to colloidal suspensions of 28 pM Au-NR@HS-Si(OH)_n (pH = 7 in aqueous solutions). In (b), WGM signal traces are shown for non-treated microresonators, and (c) shows the WGM signals after alkaline piranha treatment of the WGM microsphere.

Figure 3

Event rates for (a) Δκ and (b) Δλ obtained from time series for increasingly higher Au-NR@HS-Si(OH)_n concentrations at pH = 7 in aqueous solutions.

Figure 4

Histograms corresponding to events extracted from time series of Δκ, corresponding to increasing Au-NR@HS-Si(OH)_n concentrations explored in Figure 3: (a) 9 pM, (b) 14 pM, (c) 18 pM, and (d) 28 pM.

Figure 5

Both spike rates experimentally determined and calculated according to the Stoke–Einstein equation and Fick’s law. Experiments correspond to increasingly higher concentrations of Au-NR@HS-Si(OH)_n in pH = 7 aqueous solutions. Exposed area for PNP interaction is 1200 μm², assuming an equatorial section of 5 μm and a radius of 40 μm.

Figure 6

(a) WGM sensor traces for citrate-capped AuNR (pH 7, NaCl 10 mM, and 6 pM Au-NR), and (b) for 6 pM Au-NR@HS-Si(OH)_n. (c) Both spikes (red arrows) and steps (green arrows) are observed for citrate-capped NR at buffer with pH = 3 and 2 pM Au-NR@HS-Si(OH)_n. (d) Spike and step rate dependences on the concentration of citrate buffer at pH = 3.

Figure 7

FWHM values corresponding to pH = 3 assembled sensors (both electrostatic (red) and covalent (black) binding) after exposure to aqueous solutions with pH = 8, showing different stabilities.

Figure 8

AFM topography images obtained ex situ for samples (2 × 2 µm²; height range: 0–25 nm) modified with Au-NR@HS-Si(OH)_n, and the height for the ten considered nano-objects in the field of view.

Figure 9

(a) Background WGM shift signal for Δλ after 12 h in H₂O, (b) spike signals after adding 166 nM of C12, (c) spike rate dependence with C12 full complementary increasing concentration. (d) Histograms of the waiting times for events extracted from WGM Δλ shift signal displayed in (b) for 166 nM C12.