Katrin Kneipp

Novel optical nanosensors for probing and imaging live cells

UNLABELLEDnThis review introduces multifunctional optical nanosensors based on surface-enhanced Raman scattering (SERS) and demonstrates their application in live cells. The novel nanosensors have the potential to improve our understanding of cellular processes on the molecular level. The hybrid sensor consists of gold or silver nanoparticles with an attached reporter species. The sensor can be detected and imaged based on the SERS signature of the reporter. This results in several advantages, such as high spectral specificity, multiplex capabilities, improved contrast, and photostability. SERS sensors not only highlight cellular structures, based on enhanced Raman spectra of intrinsic cellular molecules measured in the local optical fields of the gold nanoparticles, they also provide molecular structural information on their cellular environment. Moreover, the SERS signature of the reporter can deliver information on the local pH value inside a cell at subendosomal resolution. SERS sensors are suitable for one- and two-photon excitation.nnnFROM THE CLINICAL EDITORnThis review introduces multifunctional optical nanosensors based on surface enhanced Raman scattering (SERS) and demonstrates their application in live cells. These hybrid sensors consist of gold or silver nanoparticles with an attached reporter species. The sensor can be detected and imaged based on the SERS signature of the reporter. SERS sensors highlight cellular structures and provide molecular structural information on their cellular environment. They can also deliver information on the intracellular pH-value at subendosomal resolution.

Single-Molecule SERS Spectroscopy

It has long been the dream of chemists to examine a single molecule and monitor its structural changes. The observation of a single molecule and its individual properties and structural transformations can provide useful insight into the nature of processes that cannot be studied in an ensemble of molecules due to averaging. Moreover, high-throughput structurally selective detection of single molecules and quantification of matter by counting single molecules represent the ultimate limit in chemical analysis and trace detection. Today, single-molecule studies are a topic of rapidly growing scientific and practical interest in many fields, such as chemistry, physics, life sciences, and nanotechnology [1, 2, 3]. Photons have been identified as nearly perfect tools for detecting and studying single molecules. For example, the first single-molecule detection under ambient conditions was achieved using laser-induced fluorescence [4]. However, particularly under ambient conditions, there is a limited amount of molecular information that can be obtained from a fluorescence signal. Raman spectroscopy as a vibrational technique provides a high degree of structural information but the extremely small cross section of the effect, typically ∼ 10−30 cm to 10−25 cm, with the larger values occurring only under favorable resonance Raman conditions, precludes the use of Raman spectroscopy as a method for single-molecule detection. For example, the number of Stokes photons can be estimated as the product of Raman cross section and excitation intensity: Assuming a Raman line with a scattering cross section of 10−29 cm and 100mW excitation light focused to 1μm, a single molecule scatters ∼ 10−4 photons per second, which means that one must wait more than an hour for a Stokes photon from a single molecule. In comparison, fluorescence cross sections are ∼ 10−16 cm. Estimated enhancement factors for the Raman signals in SERS started from modest factors of 10 to 10 in the initial SERS experiments [5, 6]. For excitation laser wavelengths in resonance with the absorption band of the target molecule, surface-enhanced resonance Raman scattering (SERRS)

SERS signals at the anti Stokes side of the excitation laser in extremely high local optical fields of silver and gold nanoclusters

Surface-enhanced anti-Stokes Raman scattering from pumped excited vibrational levels and surface-enhanced hyper Raman scattering show a quadratic dependence on the excitation intensity and are discussed as incoherent two-photon excited Raman processes performed in strongly enhanced local optical fields of silver- or gold nanoclusters, where both effects can experience very similar electromagnetic enhancement conditions.

Ultrasensitive and nanoscale Raman spectroscopy in biomedicine

This paper is about a new methodological approach in spectroscopy, the Raman spectroscopy, which combines the interesting optical properties of nanostructures with modern laser spectroscopy. Ultra sensitive Raman measurements at the single molecule level for biomedically relevant molecules, such as DNA fragments, neurotransmitters, proteins, as well as surface enhanced Raman scattering (SERS) studies in single living cells are reported and potential capabilities and limitations of spectroscopies exploiting local optical fields of nanostructures in the biomedical field are discussed