Adjustable Fluorescence Emission of J-Aggregated Tricarbocyanine in the Near-Infrared-II Region

Abstract

Near-infrared (NIR) J-aggregates attract increasing attention in many areas, especially in biomedical applications, as they combine the advantages of NIR spectroscopy with the unique J-aggregation properties of organic dyes. They enhance light absorption and have been used as effective biological imaging and therapeutic agents to achieve high-resolution imaging or effective phototherapy in vivo. In this work, we present novel J-aggregates composed of the well-known cyanine molecules. Cyanines are one of the few types of molecules whose absorption and emission can be shifted over a broad spectral range, from the ultraviolet (UV) to the NIR regime. They can easily transform into J-aggregates with narrow absorption and emission peaks, which is accompanied by a red shift in their spectra. In this work, we show, for the first time, that the tricarbocyanine dye (IR 820) has two sharp fluorescence emission bands in the NIR-II region with high photostability. These emission bands can be tuned to a desired wavelength in the range of 1150-1560 and 1675 nm, with a linear dependence on the excitation wavelength. Cryogenic transmission electron microscopy (cryo-TEM) images are presented, and combined with molecular modeling analysis, they confirm IR 820 π-stacked self-assembled fibrous structures.

Figure 1

Absorption and emission spectra of IR 820 in different aqueous solutions. (a) Absorption spectrum of 23 μM IR 820 in water. (b) IR 820 emission spectra in various concentrations of H2O, NaOH, NaCl, Na₂SO₄, and PBS and excitation wavelength: 810 nm. (c) Emission spectra of IR 820 dissolved in 20 and 500 mM NaCl aqueous solutions, following 685 nm excitation wavelength. The scale is logarithmic.

Figure 2

Emission spectra of IR 820 at excitation wavelengths of 660–910 nm in water. (a) 1st emission band in the range of 660–780 nm. (b) 2nd emission band in the range of 770–910 nm. (c) Dependencies of the 1st and 2nd emission peak on excitation wavelengths, each circle (1st band) or square (2nd band) represents an experiment. (d) 3rd emission band in the range of 680–825 nm.

Figure 3

Emission of IR 820 in ethanol, chloroform, and DMSO at two different excitation wavelengths (nm).

Figure 4

(a) Emission spectra of IR 820 at different concentrations of NaCl aqueous solutions at an excitation wavelength of 810 nm. (b) Correlation between the fluorescence intensities, calculated from the ratio between the peak intensity at 930 and 1217 nm.

Figure 5

Dimensions and morphology of the IR 820 in water. (a) cryo-TEM micrograph of IR 820 fibers. (b) The molecular model of the IR 820 dimer was partially energy-minimized to fit the observed dimensions.

Figure 6

Consecutive emission spectra of IR 820. (a) Fluorescence intensity of IR 820 versus time. Each colored line indicates the time of measurement from onset. (b) 1212 nm peak relative fluorescence intensity versus time. Each dot is a recorded fluorescence measurement (at time t = 0, the intensity was 100%).

Figure 7

Scheme of IR 820 energy levels (not to scale).