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The technological advancements made in optics and semiconductors (e.g., cameras and laser diodes) working with infrared have brought interest in optical bioimaging back to the forefront of research investigatingin vivomedical imaging techniques. The definition of the near-infrared transparency windows has turned optical imaging into more than just a method for topical imaging applications. Moreover, this has focused attention back to tissue fluorescence, emissions by tissues and organs that occur when excited by external illumination sources. Most endogenous fluorophores emit in the blue to green range of the electromagnetic spectrum and the resulting tissue fluorescence can be employed in studies from cells to tissue metabolism or avoided by shifting to the red if seen as unwanted autofluorescence. With the more recent move to infrared, it was discovered that autofluorescence is not limited to the visible but also strongly affectsin vivoimaging in the infrared. In this Tutorial, we give an overview on tissue fluorescence and tissue interactions with excitation light as well as their effect onin vivoimaging. Furthermore, potential sources of tissue fluorescence in the near-infrared are identified and we describe approaches for successful biomedical imaging in the biological windows, taking into consideration infrared autofluorescence and summarizing techniques for avoiding it inin vivoimaging experiments.

Ideally, any material used should be nontoxic and produced with safe, inexpensive, and energy-effective processes. In the case of optically active nanoparticles, this is often not the case, as they are frequently composed of hazardous heavy metals and/or produced with methods far from being environmentally friendly. Herein, the preparation of Ag2S-based nanoparticles via a simple green synthesis route is explored. Aqueous extracts of roasted coffee are used as sources of coordinating molecules. Optimization of the reaction conditions yields dimeric Ag���Ag2S nanoparticles, whose near-infrared photoluminescence can be switched on via H2O2-mediated oxidation. This oxidation transforms suitable photoacoustic contrast agents into fluorescence imaging probes. Theoretical calculations further clarify the role of metallic silver in determining the optical properties of Ag2S. Overall, it is demonstrated that nanomaterials with tangible applicative potential can be prepared via cost- and energy-effective synthesis strategies that entail benign, renewable chemicals.