Simple Route for Preparing Optically Trappable Probes for Surface-Enhanced Raman Scattering

Abstract

Surface-enhanced Raman scattering (SERS) has proven to be a powerful technique for a wide range of topics such as chemical analysis, materials research, and biological imaging. In this work, we report in detail on the development and characterization of micrometer-sized dielectric beads with metal colloids attached to their surface. The metalized beads were sufficiently transparent enabling optical trapping while the presence of metal islands provided the SERS. This method is fast and simple and is void of complications when compared to the Tollen's test used in previous publications. This highly efficient probe can be placed and scanned with nanometric accuracy near living cells. The process to create such probes is described including discussion of various parameters that are critical in achieving the desired results. Additionally, their use is demonstrated by detecting low quantities of a drug in aqueous solution and in a cell membrane.The enhancement of local field intensity due to local plasmon resonances near nanoscopically textured metal structures leads to many fascinating optical effects. One of them is the surfaceenhanced Raman scattering (SERS), which has been established as a powerful spectroscopic technique to chemical analysis combining high sensitivity with structural information. 1 A great deal of efforts has been made toward developing SERS substrates over the past decade. 2 Nanoparticles (NPs) with sizes around tens of nanometers have an appeal due to their larger scattering cross sections and when they are closely associated (<10 nm) intense fields emanate and contribute to the high sensitivity of the technique. Early works in developing suitable SERS surfaces concentrated on evaporation and thermal annealing of the metal to surfaces. 3,4 At a later stage, 3D hybrid structures of metal NPs on dielectric beads 5 were developed in what are now well known as nanoshells, and their applications toward SERS were initially demonstrated. 6 A simple and robust SERS scanning system can be developed with the use of optical tweezers. 7 By optically trapping a SERS active probe, advantages are gained from the combination of the high enhancement from a static configuration between the laser focus and metal surface and the scanning ability due to the in situ trapping, so that this highly efficient probe can be placed and scanned with nanometric accuracy near, for instance, living cells. However, for nanometer-sized particles, times for trapping in 3D are short. 8 For metal particles of this size, the scattering force exerted by the trapping beam overcomes the gradient force in the propagation direction.In 2002, the Tollen's test was used to create silver islands on dielectric beads and compared these surfaces to the original method. 9 The silvered beads were sufficiently transparent to allow light to refract through them, enabling the optical trapping while the presence of metal islands provided the SERS. 10 The beads made by Tollen's test 10,11 were partially covered with silver, but the homogeneity and dist...

Laser Trapping of Colloidal Metal Nanoparticles

“…107,109,[114][115] Svedberg et al reported on SERS from optically trapped silver nanoparticles (R ≈ 40 nm) functionalized by a layer of thiophenol molecules, which both act as a SERS probe and lowers the electrostatic repulsion between particles to enable aggregation. 43 Single silver particles were optically manipulated to pair up with immobilized particles, which resulted in an increase of the SERS signal by a factor of 20 or more.…”

Strategies for Optical Trapping in Biological Samples: Aiming at Microrobotic Surgeons

“…One successful approach is trapping with a laser with a frequency significantly different from the localized surface plasmon resonance . In another approach, transparent particles can be metalized to create optically trappable SERS probes …”

Optically Trapped Surface-Enhanced Raman Probes Prepared by Silver Photoreduction to 3D Microstructures

“…During the last decade, optical tweezers were successfully combined with surfaceenhanced Raman spectroscopy (SERS), a highly sensitive technique for chemical analysis. [1][2][3][4][5][6][7][8] SERS relies on local field enhancement due to plasmon resonances at nano-sized metal structures (present in colloidal suspensions 9,10 or on solid SERS substrates [11][12][13] ), which leads to manifold Raman signal enhancement from molecules located near the metal surface. [14][15][16][17] There are two approaches reported in the literature for combining optical tweezers with SERS detection.…”

“…Second, to assure transparency of the spheres (needed for successful trapping), the admissible extent of metal coverage is Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 limited. 4,7 The mentioned problems are eliminated using the microstructures proposed in this paper.…”