Comparative review of interferometric detection of plasmonic nanoparticles

Abstract

Noble metal nanoparticles exhibit enhanced scattering and absorption at specific wavelengths due to a localized surface plamson resonance. This unique property can be exploited to enable the use of plasmonic nanoparticles as contrast agents in optical imaging. A range of optical techniques have been developed to detect nanoparticles in order to implement imaging schemes. Here we review several different approaches for using optical interferometry to detect the presence and concentration of nanoparticles. The strengths and weaknesses of the various approaches are discussed and quantitative comparisons of the achievable signal to noise ratios are presented. The benefits of each approach are outlined as they relate to specific application goals.

Keywords: (120.3180) Interferometry; (160.4236) Nanomaterials; (170.1650) Coherence imaging.

Fig. 1

(a) Typical darkfield image of cell lablelled with anti-EGFR gold nanoparticles (b) Scattering spectrum for cell shown in (a). General scheme for darkfield microspectroscopy includes a light source, detector, darkfield optics and wavelength selection at either the illumination or detection section.

Fig. 2

Illustration of photothermal absorption by nanoparticles. A heating beam matched to the nanoparticle resonance wavelength deposits thermal energy, which in turn creates a local refractive index change as the heat dissipates into the surrounding medium. The local refractive index change can be measured with an imaging scheme sensitive to optical phase. Sinusoidal modulation of the heating beam allows lock-in detection and a subsequent improvement in SNR. Because heat deposition is potentially damaging for biological samples, photothermal detection methods that achieve high SNR while minimizing the amount of heating power required are attractive options for functional imaging.

Fig. 3

(A) Photothermal OCT system used by Kim, et al. The photothermal excitation lasers (405nm and 532nm) are matched to the excitation peaks of silver and gold nanoparticles, respectively. The OCT system uses illumination from a superluminescent diode with a bandwidth of 50nm, centered at 830nm. (B) Experimental sample geometry. Two coverslips sandwich a 140μm-thick agar matrix with embedded nanoparticles. A-scans are captured at 35kHz. Amplitude peaks arising from surfaces 1 and 2 are identified, and the phase difference between them is computed. (C) Transient photothermal phase response to single excitation pulses. (D) Photothermal phase response to square-wave periodic excitation. Note that the exponential rise corresponds to heat buildup that is not fully dissipated after each excitation period, while the high-frequency modulation corresponds to the periodic heating induced by the periodic excitation. (E) Multiplexed nanoparticle detection. The 405nm and 532nm lasers are modulated at 200Hz and 1 kHz, respectively. The phase profile seen in the inset is Fourier transformed to yield distinct peaks corresponding to the heating responses of each nanoparticle species. Figure adapted from Kim, et al. [42]

Fig. 4

Experimental Setup for combination darkfield and photothermal imaging. Light from a HeNe 633nm source is split into a reference and probe arms. The probe arm illuminates the sample and interferes with the reference arm which is incident at an angle onto the camera to form an off axis hologram. Both arms pass through identical objective lenses. A second, 532nm Nd:Yag laser is used as a heating beam. A beam expander is used to control the heating beam diameter in order to selectively illuminate the sample. A 532nm notch filter is placed in front of the camera to filter out the 532nm green beam. The LED ring was used for dark field illumination.

Fig. 5

The phase change as a function of nanoparticle concentration for three different photothermal media: Water, Glycerol and Glycerol Ethanol 50%. The slopes of the linear fit curves are1.32 radians/1011⋅particlesmL,3.4 radians/1011⋅particlesmL _{9.76 radians/1011⋅particlesmL} for Water, Glycerol and Glucerol Ethanol 50% respectively.

Fig. 6

Comparative images of a single nanoparticle. (Main) Dark field image of a field of 60 nm gold nanoparticles adhered to a glass coverglass using silane and immersed in glycerol/ethanol mixture. (Top inset) A crop of the dark field image centered on the heating beam location. The circle indicates the full width half maximum of the heating beam with nanoparticle present. (Bottom inset) Phase change where the heating beam is incident on the sample, arrow indicates nanoparticle signature.