True FRET‐Based Sensing of pH via Separation of FRET and Photon Reabsorption

Abstract

Among the most promising sensors being developed are those that rely on Förster resonance energy transfer (FRET). FRET is a non-radiative process where energy can be transferred from a donor (ion or molecule) to an acceptor. [3][4][5] The rate of this exchange is highly distance-dependent and fades rapidly with increasing donoracceptor separation. Thus sensors that utilize FRET are capable of measuring local variations in critical biological parameters.Of the possible FRET-based sensors, those based on upconverting nanoparticles (UCNPs) are among the most promising. UCNPs are unique nanoparticles that can emit at several distinct visible wavelengths from a single, near-infrared excitation source. UCNPs exhibit many advantageous characteristics for bioimaging and sensing such as no photobleaching, no blinking, and no background autofluorescence. [6,7] In particular, many groups have attempted to utilize UCNPs as a FRET-based sensor by conjugating a fluorescent dye to the UCNP surface. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] The key advantage of this configuration over other FRET-based schemes is that the multimodal emission of UCNPs allows for ratiometric sensing in which the relative intensities of two distinct peaks are compared rather than just the enhancement or quenching of a single one. Such ratiometric sensing reduces the sensing error due to variations in excitation intensity or nanoparticle concentrations, and thus makes these sensors much more robust against both sensor inhomogeneity and environmental noise. [26] However, FRET is not the sole process occurring that can affect the collected emissions. Unless a single UCNP can be measured directly, there exists a non-negligible chance that a photon emitted by one UCNP can be absorbed by a dye molecule on another nanoparticle. Such photon reabsorption (PR) would in turn lead to a decrease in the collected emission, thereby obscuring the true, localized FRET response of the original nanoparticle. An additional complication also arises from the fact that many of the visible UCNP emission lines originate from the same energy level. Since FRET acts as a decay pathway for an excited state electron, all emissions from this level should be affected equally by FRET, not just the one that overlaps with the dye's absorption band. To our knowledge, although the distinction between FRET and photon reabsorption has been discussed before, [27][28][29] no FRET-UCNP sensor has yet been proposed that accurately accounts for these complications.

A scalable fabrication method for gold nanodisk-upconverting nanoparticle hybrid nanostructures

“…The first stage begins with the synthesis of NaYF 4 :Yb 3+ ,Tm 3+ UCNPs using the well-established thermal coprecipitation method. 4,37,38 The as-synthesized UCNPs exhibited a narrow size distribution, with an average diameter of 32.6 nm and a standard deviation of only 1.6 nm (Fig. S1 †).…”

“…Upconverting nanoparticles (UCNPs) have been at the center of many interesting applications in bioimaging, 1,2 sensing, 3,4 drug delivery, 5,6 and optogenetics. 7,8 They are typically composed of an inorganic host crystal such as NaYF 4 that is doped with rare-earth (RE) ions such as Yb 3+ , Er 3+ , and Tm 3+ .…”

“…This simple demonstration highlights the potential of this nanofabrication method to produce stimuli-responsive gold-UCNP hybrid nanostructures for biomedical imaging and sensing applications. 4 F was separately prepared. The ODE/OA solution was cooled down to 50 °C, opened to atmosphere, and the methanol solution was added dropwise.…”

Upconversion Nanoparticles Based Sensing: From Design to Point‐of‐Care Testing

“…Modulating the spectral overlap can be achieved by inducing a reaction between the dye indicators and the sensing targets, resulting in the creation of new absorption peaks that overlap with the UCNPs' emission peaks. [88] Alternatively, weakening or eliminating old absorption peaks that overlap with the UCNPs' emission peaks can also effectively modify the spectral overlap. [89,90] In the strategy of controlling the RET process by adjusting spectral overlap, the dye indicators in UCNPs sensors serve dual roles as both energy acceptors and target recognition elements.…”