Ultralarge Modulation of Fluorescence by Neuromodulators in Carbon Nanotubes Functionalized with Self-Assembled Oligonucleotide Rings

Abstract

Non-covalent interactions between single-stranded DNA (ssDNA) oligonucleotides and single wall carbon nanotubes (SWNTs) have provided a unique class of tunable chemistries for a variety of applications. However, mechanistic insight into both the photophysical and intermolecular phenomena underlying their utility is lacking, resulting in obligate heuristic approaches for producing ssDNA-SWNT based technologies. In this work, we present an ultrasensitive “turn-on” nanosensor for neuromodulators dopamine and norepinephrine with strong relative change in fluorescence intensity (ΔF/F0) of up to 3500%, a signal appropriate for in vivo neuroimaging, and uncover the photophysical principles and intermolecular interactions that govern the molecular recognition and fluorescence modulation of this nanosensor synthesized from the spontaneous self-assembly of (GT)6 ssDNA rings on SWNTs. The fluorescence modulation of the ssDNA-SWNT conjugate is shown to exhibit remarkable sensitivity to the ssDNA sequence chemistry, length, and surface density, providing a set of parameters with which to tune nanosensor dynamic range and strength of fluorescence turn-on. We employ classical and quantum mechanical molecular dynamics simulations to rationalize our experimental findings. Calculations show that (GT)6 ssDNA form ordered rings around SWNT, inducing periodic surface potentials that modulate exciton recombination lifetimes. Further evidence is presented to elucidate how dopamine analyte binding modulates SWNT fluorescence. We discuss the implications of our findings for SWNT-based molecular imaging applications.

High Throughput Evolution of Near Infrared Serotonin Nanosensors

“…ssDNA-SWNT constructs whose polymer corona phase recognizes 5-HT were evolved from an initial 18-mer nucleotide library of > 10 10 unique sequences by inducing competitive complexation between ssDNAs and SWNTs in the presence of 5-HT, through a process we termed SELEC. Prior studies have shown that analytes that induce selective fluorescence modulation of SWNTs also stabilize the ssDNA corona 25 , thus we reason that evolved 5-HT recognizing sequences may preferentially populate the evolved ssDNA pool via stabilization of the 5-HT-ssDNA-SWNT constructs. Conversely, the control SELEC library evolved in the absence of 5-HT should contain ssDNA sequences with high affinity for the SWNT surface but not specifically for 5-HT.…”

“…We further demonstrated that surface-immobilized SWNT, without the ssDNA coating, do not respond to 5-HT, confirming the molecular recognition role of the ssDNA polymer when adsorbed to the SWNT surface ( Figure S12). Importantly, prior studies of ssDNA-SWNT nanosensors have confirmed ms-scale nanosensor reversibility kinetics 25,27 , the absence of photobleaching 38 , and in vivo compatibility 39 , suggesting nIRHT could serve as an optical probe for long-term endogenous 5-HT imaging in the brain.…”

“…Prior work has demonstrated that semiconducting near-infrared fluorescent SWNT, when noncovalently functionalized by amphiphilic polymers such as ssDNA, can generate synthetic molecular recognition elements on the SWNT surface. The resulting ssDNA-SWNT moiety can then selectively bind a molecular analyte for its optical detection [23][24][25][26] . The process of generating ssDNA-SWNT nanosensor candidates, and their screening against target analytes, has since revealed optical nanosensors for neurotransmitters including a (GT)6-SWNT nanosensor for dopamine 25 which was subsequently used to image dopamine dynamics in the brain striatum and capture the influence of dopamine receptor drugs on dopamine dynamics at the level of individual synapses 27 .…”

“…The resulting ssDNA-SWNT moiety can then selectively bind a molecular analyte for its optical detection [23][24][25][26] . The process of generating ssDNA-SWNT nanosensor candidates, and their screening against target analytes, has since revealed optical nanosensors for neurotransmitters including a (GT)6-SWNT nanosensor for dopamine 25 which was subsequently used to image dopamine dynamics in the brain striatum and capture the influence of dopamine receptor drugs on dopamine dynamics at the level of individual synapses 27 . Despite the nascent utility of SWNT-based nanosensor technology, the nanosensor screening process involves a heuristic approach in which each nanosensor candidate must be synthesized individually before screening, limiting candidate nanosensor libraries to a few dozen, thus restricting the throughput of nanosensor discovery.…”

“…The 6-mer (C)6 sequence was introduced because this sequence was previously shown to have a high affinity for the SWNT surface 29 , and could thus serve as a SWNT-binding spacer sequence to separate the random 18mer sequence from the PCR primer sequences. This ssDNA library was used to suspend SWNT using a previously described sonication protocol 25 , to create a SWNT suspension with 6.9 × 10 10 unique ssDNA sequences for both an experimental and control ssDNA-SWNT sample: our experimental sample was generated in PBS solution by sonicating 10 µg SWNT with 2.8 mg ssDNA and 100 nmol of 5-HT. Our control sample was generated similarly but without 5-HT.…”