Comparative review of interferometric detection of plasmonic nanoparticles

Reflected light microspectroscopy for single-nanoparticle biosensing

Conventional and dark-field microscopy in the transmission mode is extensively used for single plasmonic nanoparticle (NP) imaging and spectral analysis. However, application of the transmission mode for realtime biosensing to single NP poses strict limitations on the size and material properties of the microfluidic system. This article proposes a simple optical technique based on reflected light microscopy to perform microspectroscopy of a single NP placed in a conventional, nontransparent liquid delivery system. The insertion of a variable spot diaphragm in the optical path reduces the interference effect that occurs at the NP-substrate interface and improves the signal-to-noise ratio in NP imaging. Using this method, we demonstrated spatial imaging and spectral analyses of 60-, 80-, and 100-nm single gold NPs. A single-NP sensor based on a 100-nm NP was used for real-time measurement of bulk refractive index changes in the microfluidic channel and for detection of fast dynamic poly(ethylene glycol) attachment to the NP surface. Finally, electrochemical single-particle microspectroscopy was demonstrated by using a methylene blue electroactive redox tag. The proposed optical approach is expected to significantly improve the miniaturization and multiplexing capabilities of high-throughput biosensing based on single NP.

Fast wide-field photothermal and quantitative phase cell imaging with optical lock-in detection

We present a fast, wide-field holography system for detecting photothermally excited gold nanospheres with combined quantitative phase imaging. An interferometric photothermal optical lock-in approach (POLI) is shown to improve SNR for detecting nanoparticles (NPs) on multiple substrates, including a monolayer of NPs on a silanized coverslip, and NPs bound to live cells. Furthermore, the set up allowed for co-registered quantitative phase imaging (QPI) to be acquired in an off-axis holographic set-up. An SNR of 103 was obtained for NP-tagging of epidermal growth factor receptor (EGFR) in live cells with a 3 second acquisition, while an SNR of 47 was seen for 20 ms acquisition. An analysis of improvements in SNR due to averaging multiple frames is presented, which suggest that residual photothermal signal can be a limiting factor. The combination of techniques allows for high resolution imaging of cell structure via QPI with the ability to identify receptor expression via POLI.

Introduction: feature issue on optical molecular probes, imaging, and drug delivery

The editors introduce the Biomedical Optics Express feature issue "Optical Molecular Probes, Imaging, and Drug Delivery," which is associated with a Topical Meeting of the same name held at the 2013 Optical Society of America (OSA) Optics in the Life Sciences Congress in Waikoloa Beach, Hawaii, April 14-18, 2013. The international meeting focused on the convergence of optical physics, photonics technology, nanoscience, and photochemistry with drug discovery and clinical medicine. Papers in this feature issue are representative of meeting topics, including advances in microscopy, nanotechnology, and optics in cancer research.