processes to destroy microorganisms. The concentration of the released substances changes spatially and temporally and is a biochemical fingerprint of the biological state. However, such biologically relevant processes occur on temporal (ms) and spatial (nm) scales that are difficult to access using established methods. [3] For example, electrochemical methods, such as amperometry or voltammetry, lack the required spatial resolution determined by the number of electrodes and are invasive as the microelectrodes penetrate the tissue. [4] On the other hand, optical methods provide often only indirect information, for example, by labeling cellular components [5,6] or suffering from photobleaching. [7] In this context, nanomaterials, such as single-wall carbon nanotubes (SWCNTs), have emerged as promising building blocks to capture these dynamics. [3,8] In addition to a high surface-to-volume ratio that makes them sensitive to single-molecule detection, [9][10][11][12] their surface can be chemically tailored. [13][14][15][16][17] Thus, SWCNTs have already been used for several bioimaging studies [18,19] and the detection of numerous analytes such as reactive oxygen species, [20][21][22][23] small molecules like nitroaromatics [24,25] or neurotransmitters, [26,27] proteins, [28][29][30] sugars, [31] enzymes, [32] or bacteria. [33] Due to their fluorescence in the near-infrared (NIR, 850-1700 nm), which shows no bleaching or blinking, they represent stable fluorophores, whose emission falls within the biological transparency window. [34] Here, Biochemical processes are fast and occur on small-length scales, which makes them difficult to measure. Optical nanosensors based on singlewall carbon nanotubes (SWCNTs) are able to capture such dynamics. They fluoresce in the near-infrared (NIR, 850-1700 nm) tissue transparency window and the emission wavelength depends on their chirality. However, NIR imaging requires specialized indium gallium arsenide (InGaAs) cameras with a typically low resolution because the quantum yield of normal Si-based cameras rapidly decreases in the NIR. Here, an efficient one-step phase separation approach to isolate monochiral (6,4)-SWCNTs (880 nm emission) from mixed SWCNT samples is developed. It enables imaging them in the NIR with high-resolution standard Si-based cameras (>50× more pixels). (6,4)-SWCNTs modified with (GT) 10 -ssDNA become highly sensitive to the important neurotransmitter dopamine. These sensors are 1.7× brighter and 7.5× more sensitive and allow fast imaging (<50 ms). They enable high-resolution imaging of dopamine release from cells. Thus, the assembly of biosensors from (6,4)-SWCNTs combines the advantages of nanosensors working in the NIR with the sensitivity of (Si-based) cameras and enables broad usage of these nanomaterials.

Co x Fe 3−x O 4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co 2+ and Fe 3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, Co x Fe 3−x O 4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperatureresolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles' surface.

Biochemical processes are fast and occur on small length scales, which makes them difficult to measure. Optical nanosensors based on single-wall carbon nanotubes (SWCNTs) are able to capture such dynamics. They fluoresce in the near-infrared (NIR, 850 3 1700 nm) tissue transparency window and the emission wavelength depends on their chirality. However, NIR imaging requires specialized and cooled InGaAs cameras with low resolution because the quantum yield of normal Si-based cameras rapidly decreases in the NIR. Here, we developed an efficient one-step phase separation approach to isolate monochiral (6,4)-SWCNTs (880 nm emission) from mixed SWCNT samples. It enabled us to image them in the NIR with highresolution standard Si-based cameras (>50 x more pixels). (6,4)-SWCNTs modified with (GT)10-ssDNA become highly sensitive for the important neurotransmitter dopamine. These sensors are 1.7-fold brighter and 7.5 x more sensitive and allow fast imaging (< 50 ms). They enable high-resolution imaging of dopamine release from cells. Thus, the assembly of biosensors from (6,4)-SWCNTs combines the advantages of nanosensors working in the NIR with the sensitivity of (Si-based) cameras and enables broad usage of these nanomaterials.