Dyes for biological second harmonic generation imaging

Abstract

Nonlinear optical imaging has revolutionized microscopy for the life sciences. Second harmonic generation (SHG), the younger sibling of two-photon excited fluorescence (2PF), is a technique that can produce high resolution images from deep inside biological tissues. Second harmonic light is generated by the coherent scattering of an ensemble of aligned chromophores in a focused, pulsed laser beam. SHG is only generated at the focal spot, reducing the background signal, and requires ordered chromophores, so is highly structure-specific. In contrast to two-photon fluorescence, the physical process that creates the signal does not require the formation of excited states, allowing elimination of harmful photochemistry. While the SHG of native proteins and biopolymers is well known, the use of exogenous dyes can provide SHG contrast from areas without a sufficiently high intrinsic quadratic hyperpolarizability, β. Dyes for SHG primarily target lipid bilayers; a trait that, combined with sensitivity to transmembrane potential, allows monitoring of action potentials in a variety of excitable cells, most importantly mammalian neurons. This article summarizes the principles of SHG imaging and explores approaches for maximizing the SHG signal from a biological specimen. We survey methods of optimizing the optical set-up, enhancing the β of the dye and achieving biological compatibility. In conclusion, we examine novel applications of SHG imaging and highlight promising directions for the development of the field.

Porphyrins for Probing Electrical Potential Across Lipid Bilayer Membranes by Second Harmonic Generation

“…[3,6,19,30] Our results for FM4-64, di-4-ANEPPS, and RH237 are broadly in line with previously published studies on these dyes, although this is the first time that all three dyes have been compared under identical conditions in HLBs. FM4-64 has been reported to give SHG responses in the range DS/S per 100 mV = 5-15 %, [17][18][19][31][32][33] which is slightly larger than our value (ca.…”

“…[45][46][47][48][49][50][51] Conjugated porphyrin dimers exhibit much larger third-order susceptibilities (g) than the corresponding porphyrin monomers, [49][50][51] thus Equation (3) implies that dimeric analogues of JR1 will display even higher sensitivities to electric field. Previous studies on voltagesensitive SHG dyes have focused entirely on styryl and retinal chromophores.…”

“…[1,2] Second harmonic generation (SHG) is one of the most promising methods for imaging membrane potential, although so far this technique has only been demonstrated with a narrow range of dyes. [3] Here we show that SHG from a porphyrin-based membrane probe gives a fast electro-optic response to an electric field which is about 5-10 times greater than that of conventional styryl dyes. Our results indicate that porphyrin dyes are promising probes for imaging membrane potential.…”

“…[22] The main disadvantage of SHG is that it often gives lower intensity than fluorescence; there is a need for the design of brighter, more voltage-sensitive SHG dyes. [3,23] Styryl dyes and retinal chromophores have been shown to exhibit voltage-sensitive SHG when localized in plasma membranes, [15,24] and the voltage sensitivity of their SHG is greater than that of their fluorescence. [19][20][21] Recently we reported that amphiphilic porphyrins such as JR1 exhibit strong SHG when localized in lipid membranes.…”

“…[4] Fluorescent voltage-sensitive dyes were first developed 30 years ago, [5] and recent advances in the molecular design of these dyes [6][7][8][9][10] have made it possible to track the spatial evolution of action potentials. [11][12][13][14] Compared with fluorescence, SHG imaging has several advantages as a technique for probing membrane potential: [3,[15][16][17][18][19][20][21] SHG has a greater intrinsic sensitivity to electric fields than fluorescence, [19][20][21] and it is only produced by noncentrosymmetric molecules in noncentrosymmetric environments, thus making it ideal for probing interfaces such as lipid bilayers. Similar to twophoton-excited fluorescence, SHG is a two-photon nonlinear optical effect, and it brings the advantages of depth penetration associated with multiphoton microscopy.…”