Lengthening the Lifetime of Common Emissive Probes to Microseconds by a Jigsaw‐Like Construction of NIR‐Responsive Nanohybrids

Abstract

Chemical/biological sensing by means of luminescence still requires improvement of signal contrast and depth of sample imaging. Though there is a considerable number of emissive molecular and quantum dot probes, most of them absorb in the UV–vis range and emit in the visible and exhibit emission lifetime in the nanoscale. Optical bioimaging in the near infrared window based on these dyes offers reduced light scattering, lower competitive absorption, and reduced background autofluorescence, but it requires a high‐power pulsed laser. An approach to lengthening the emission lifetime of common fluorophores/probes is presented. It consists of coupling them to lanthanide‐based upconversion nanoparticles (UCNPs) via an easy, jigsaw‐like strategy. As a proof of concept, nanohybrids comprising UCNPs attached to fluorescein (a well‐known pH sensor) are prepared by connecting the key pieces through ionic interactions, i.e., without using any other reagents. Fluorescence imaging of the nanohybrids clearly shows the co‐occurrence of the UCNP and the chemical probe and as a consequence the long‐lived fluorescence of the probe due to resonance energy transfer from the UCNP. This strategy opens up endless possibilities to prepare new nanohybrids by selecting the key jigsaw pieces to exploit the color emission of common probes.

NIR laser scanning microscopy for photophysical characterization of upconversion nanoparticles and nanohybrids

“…However, we have previously demonstrated how this drawback is, in fact, advantageous for obtaining information about the evolution of the UCNP emission over time. 30,32,46 The time-resolved NIR-LSM principle can be clearly explained by imaging an agglomerate of UC Tm @Rh110 at 4 μs per pixel dwell time. The image of Fig.…”

“…[30][31][32] Nanohybrids can be designed to lengthen the photoluminescence of a nearby acceptor ( photoactive NPs, organic fluorophores) via resonant energy transfer (RET) from the UCNPs, which is of interest in (bio)sensing, photodynamic therapy, and bioimaging applications, and photonics. [30][31][32] Previous studies performed by our group have demonstrated that when the RET process is highly efficient, only dots (instead of tails) are observed upon 975 nm excitation of the UCNHs due to a drastic quenching of the lifetime of the UCNPs. 30 On the other hand, for moderately efficient RET, a shortening of the tail of the upconversion emission, together with a lengthening in the fluorophore lifetime, can be observed.…”

“…30 On the other hand, for moderately efficient RET, a shortening of the tail of the upconversion emission, together with a lengthening in the fluorophore lifetime, can be observed. 32 Here, as an example to illustrate the visualization of a resonant energy transfer process by using this technique, an UCNH consisting of rhodamine 110 (Rh110) grafted onto UC Tm was built. Rh110 absorption overlaps with the 475 nm emission band of UC Tm (Fig.…”

“…29, ‡ Our group has demonstrated the possibility of using the NIR-LSM as a technique for the determination of "empirical" UCNP emission lifetimes. [30][31][32] The ability of the NIR-LSM to scan samples at different scan speeds by controlling the dwelltime has proved to be an appropriate solid-state approach to measure such decay times. The key step of the procedure is the transformation of the space-resolved image to a time-resolved profile, from which the empirical decay lifetime of the emissions can be obtained.…”

“…This approach has also been useful to detect the lengthening of the organic dye emission present in upconversion nanohybrids as a consequence of an energy transfer process from the UCNP emissive lanthanide to the organic dye. 31,32 In this paper, recent findings with novelties only touched upon in previous works are gathered together and the scope of the NIR-LSM technique is explored in much more detail to offer a complete vision of the versatility of the NIR-LSM technique which is extremely useful for the photophysical characterization of long-lived emissive (nano)hybrids. To this end, four samples of UCNPs have been prepared with different types and concentration of dopants, namely oleate-capped NaYF 4 :Yb 3+ (20%),Er 3+ (2%),UC Er,2 ; NaYF 4 :Yb 3+ (20%),Er 3+ (20%), UC Er,20 ; NaYF 4 :Yb 3+ (20%),Tm 3+ (0.1%),UC Tm ; and NaYF 4 : Yb 3+ (20%),Ho 3+ (1%),UC Ho .…”