Infinity 2

A customized microscope (Olympus BX51) with a halogen white light lamp, a charge-coupled device camera (Infinity 2), spectrometer (Ocean Optics QE65000), and a 100 $\times$ objective (NA 0.8 Olympus LMPLFLN) was used to capture optical DF images and spectra. All the DF spectra were integrated for 1 s and normalized using white light scattering of a standard diffuser. SERS spectra were recorded on the same microscope with a 633 nm laser for excitation, a 100 $\times$ objective (NA 0.9 Olympus LWD) and an Andor Newton EMCCD camera coupled to a Horiba Triax 320 spectrometer. For non-electrochemical SERS measurements on as-synthesized eNPoMs (in air), the laser power was set at 6 $\mu \mathrm{W}$ on 2 nm shell samples, and 3 $\mu \mathrm{W}$ on all other shell thickness NPoMs, for optimal signal-to-noise ratios and minimization of picocavity events. For electrochemical SERS measurements, the laser power was set to 10 $\mu \mathrm{W}$ for all samples. The integration time of all SERS measurements was 1 s if not otherwise specified. For non-electrochemical DF and SERS measurements, automated scans were performed using customized particle-tracking Python code.

Optical dark-field
(DF) images and spectra of samples are acquired using a charge-coupled-device
camera (Infinity 2) and spectrometer (Ocean Optics QE65000) with numerical
aperture (NA) 0.8 100× objectives (Olympus LMPLFLN) in a customized
microscope (Olympus BX51). A halogen lamp is used as the white light
source. SERS measurements are recorded using the same microscope coupled
to a 633 nm laser at 100 μW with 1s integration time, and a
0.9 NA Olympus LWD objective for both excitation and collection. Spectra
are recorded by an Andor Newton EMCCD camera coupled to a Horiba Triax
320 spectrometer.

Optical BF/DF images and spectra of samples were obtained using a CCD camera (Infinity 2) and spectrometer (Ocean Optics QE65000) with 100 × objectives (Olympus MPlanFLN) in a customized microscope (Olympus BX51). A halogen lamp (Philip 7023) was used as the white light source. Reflection and transmission measurements were conducted in BF configuration, and scattering measurements were conducted in DF configuration.