Protection of silver and gold LSPR biosensors in corrosive NaCl environment by short alkanethiol molecules; characterized by extinction spectrum, helium ion microscopy and SERS

Abstract

We investigated the utility of localized surface plasmon resonance sensors in a biologically relevant environment containing NaCl. Our sensors are fabricated by depositing gold or silver on a monolayer of adsorbed monodisperse SiO₂ nanospheres. While silver nanostructures are rather unstable in air and water as assessed by drifts in the extinction peak, even gold nanostructures have been found to drift at elevated NaCl concentrations. In an attempt to protect these nanostructures against NaCl, we modified them with alkanethiols with different lengths in the vapor phase and found that shorter chain alkanethiols such as 1-butanethiol are particularly effective against even 250 mM NaCl, rather than longer-chain alkanethiols more suitable for robust SAM formation. A vapor phase treatment method, in contrast to widely used solution phase treatment methods, was selected with the intention of reducing the solvent effect, i.e. destruction of intricate nanostructures upon contact with a solvent when nanostructures have been prepared in a vacuum system. Moreover, the treatment with 1-butanethiol led to an enhanced sensitivity as expressed by peak shift in nm per refractive index unit, nm per RIU. We show the results of evaluating alkanethiol-protected silver and gold nanostructures by extinction spectroscopy, helium ion microscopy and surface-enhanced Raman spectroscopy. The vapor phase treatment method with short chain alkanethiols is an effective way to protect intricate gold and silver nanostructures prepared in a vacuum system.

Fig. 1. Extinction spectra of random Au- and AgFON LSPR sensors prepared with 150 nm SiO₂ nanospheres with 17 and 22 nm thick top layers. HIM images are of nanospheres covered by 16.2 and 20.1 nm thick Au.

Fig. 2. The extinction peak wavelength as a function of the refractive index for AuFON LSPR sensors. Results from sensors with 17 and 22 nm thick top layers are shown for comparison.

Fig. 3. Time-course changes of the extinction spectra of untreated Au- and AgFON LSPR sensors during the first 5 min, with a 20 s time interval. (a) Immersion of a AuFON LSPR sensor into PBS. (b) Immersion of a AuFON LSPR sensor into 250 mM NaCl solution. (c) Immersion of a AgFON LSPR sensor into PBS. (d) Immersion of a AgFON LSPR sensor into 250 mM NaCl solution.

Fig. 4. Time-course changes of the extinction spectra of the protected Au- and AgFON LSPR sensors during the initial 5 min, with a 20 s time interval. (a) Immersion of the AuFON LSPR sensor into PBS. (b) Immersion of the AuFON LSPR sensor into 250 mM NaCl solution. (c) Immersion of the AgFON LSPR sensor into PBS. (d) Immersion of the AgFON LSPR sensor into 250 mM NaCl solution.

Fig. 5. HIM images showing morphology of AgFON LSPR sensors upon 5 min immersion to 250 mM NaCl solution. With the 1-BT treatment, there is little change in morphology before (a) and after (b). Without the treatment, nanostructures undergo a drastic change, from (c) to (d). The insets are photographs of these sensors.

Fig. 6. The extinction peak wavelength as a function of the refractive index for 1-BT treated Au- and AgFON LSPR sensors, with 17 and 22 nm thick top layers.

Fig. 7. Response of a 1-BT treated sensor as a biosensor. The sensor was first incubated in the primary antibody solution for a few minutes, followed by rinsing with PBS. Subsequently, 30 μL of the secondary antibody solution was pipetted in gently; after dilution, the final concentration was reduced by half.

Fig. 8. SERS spectra of 1-BT, 1-HT and 1-OT after 30 min exposure to their vapors. The inset shows Raman spectra of liquid 1-BT held in a glass capillary and an empty glass capillary, and a SERS spectrum of 1-BT in the S–H stretch region.

Fig. 9. The 1-BT adsorption process evaluated by time-course changes in SERS measurements. The inset shows time-course changes of the 700 cm⁻¹ and 1100 cm⁻¹ peaks.