Hybrid Upconversion Nanomaterials for Optogenetic Neuronal Control

Abstract

Nanotechnology-based approaches offer the chemical control required to develop precision tools suitable for applications in neuroscience. We report a novel approach employing hybrid upconversion nanomaterials, combined with the photoresponsive ion channel channelrhodopsin-2 (ChR2), to achieve near-infrared light (NIR)-mediated optogenetic control of neuronal activity. Current optogenetic methodologies rely on using visible light (e.g. 470 nm blue light), which tends to exhibit high scattering and low tissue penetration, to activate ChR2. In contrast, our approach enables the use of 980 nm NIR light, which addresses the short-comings of visible light as an excitation source. This was facilitated by embedding upconversion nanomaterials, which can convert NIR light to blue luminescence, into polymeric scaffolds. These hybrid nanomaterial scaffolds allowed for NIR-mediated neuronal stimulation, with comparable efficiency as that of 470 nm blue light. Our platform was optimized for NIR-mediated optogenetic control by balancing multiple physicochemical properties of the nanomaterial (e.g. size, morphology, structure, emission spectra, concentration), thus providing an early demonstration of rationally-designing nanomaterial-based strategies for advanced neural applications.

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Nanoscale
COMMUNICATION
Cite this: DOI: 10.1039/c5nr03411f
Received 24th May 2015,
Accepted 12th September 2015
DOI: 10.1039/c5nr03411f
www.rsc.org/nanoscale
Hybrid upconversion nanomaterials for
optogenetic neuronal control†
Shreyas Shah,
a
Jing-Jing Liu,
b
Nicholas Pasquale,
a
Jinping Lai,
a
Heather McGowan,
b
Zhiping P. Pang*
b
and Ki-Bum Lee*
a
Nanotechnology-based approaches offer the chemical control
required to develop precision tools suitable for applications in
neuroscience. We report a novel approach employing hybrid
upconversion nanomaterials, combined with the photoresponsive
ion channel channelrhodopsin-2 (ChR2), to achieve near-infrared
light (NIR)-mediated optogenetic control of neuronal activity.
Current optogenetic methodologies rely on using visible light
(e.g. 470 nm blue light), which tends to exhibit high scattering and
low tissue penetration, to activate ChR2. In contrast, our approach
enables the use of 980 nm NIR light, which addresses the short-
comings of visible light as an excitation source. This was facilitated
by embedding upconversion nanomaterials, which can convert
NIR light to blue luminescence, into polymeric scaffolds. These
hybrid nanomaterial scaffolds allowed for NIR-mediated neuronal
stimulation, with comparable efficiency as that of 470 nm blue
light. Our platform was optimized for NIR-mediated optogenetic
control by balancing multiple physicochemical properties of the
nanomaterial (e.g. size, morphology, structure, emission spectra,
concentration), thus providing an early demonstration of ration-
ally-designing nanomaterial-based strategies for advanced neural
applications.
The mammalian brain is a phenomenal piece of ‘organic
machinery’, consisting of an intricate network of neurons dis-
persed in a mixture of chemical and biochemical constituents.
While remarkable advances have been made to better under-
stand the brain, the intrinsic complexity of the brain makes it
difficult to probe the innate neuronal connectivity and to
further modulate this activity. This is partly due to challenges
in developing tools to effectively interact with the type of
neurocircuitry found in the brain. Yet, given that the biomole-
cular interactions and chemical communication in the brain
occur at the nanoscale, there is great interest in leveraging
advances in nanotechnology for neuroscience research.
1
The
reproducible control of chemical reactions and advanced
nanoscience have enabled the generation of numerous types of
nanomaterials and nanostructures with well-defined compo-
sitions, shapes and properties. This offers tremendous
promise for creating a seamless integration of neural cells with
synthetic materials, which can be further exploited to achieve
improved control of neuronal activity and function. In this
way, nanotechnology-based methods can be rationally-
designed to interface with neural systems and to address
prevalent challenges in neuroscience.
Herein, we demonstrate a unique application of inorganic–
organic hybrid nanomaterials to the field of optogenetics.
Optogenetics is a revolutionary technology that has trans-
formed the field of neuroscience, allowing for the optical
control of neuronal activity by using light (“opto”) and gene-
tically-encoded photosensitive proteins (“genetics”).
2,3
In the
last decade, optogenetic approaches have been successfully uti-
lized to explore numerous neural states and disorders includ-
ing fear, anxiety, addiction, reward-seeking behavior, autism
and Parkinson’s disease.
4
While it has facilitated novel investi-
gations that were previously infeasible, the efficient opto-
genetic manipulation of neural activity is contingent on
delivering a sufficient dose of light to the target neuronal
cells.
5
Visible light (∼400–600 nm) has been primarily used in
this regard to stimulate the photoresponsive opsin proteins
(e.g. channelrhodopsins, ChR, that are activated by 470 nm
blue light). A number of groups have also attempted to achieve
optogenetic control using other wavelengths of illumination
such as red-shifted light sources (>600 nm),
6,7
which can offer
opportunities for multiplexed stimulation. The most common
approach to implement this strategy has involved the combi-
nation of genome-wide screening and complex molecular
engineering to generate new variants of microbial opsin that
are sensitive to red light.
8
However, such a strategy tends to
require balancing numerous opsin-dependent design consider-
ations including spectral specificity, optimal expression in
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5nr03411f
a
Department of Chemistry & Chemical Biology, Rutgers, The State University of New
Jersey, Piscataway, NJ, USA. E-mail: kblee@rutgers.edu; http://kblee.rutgers.edu/;
Fax: +1 732-445-5312; Tel: +1 848-445-2081
b
Child Health Institute of New Jersey and Department of Neuroscience and Cell
Biology, Rutgers-Robert Wood Johnson Medical School, New Brunswick, NJ, USA.
E-mail: pangzh@rwjms.rutgers.edu; Fax: +1 732-235-8612; Tel: +1 732-235-8074
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target cells, efficient cellular trafficking to the plasma mem-
brane and optimal kinetics/ion specificity.
9
While valuable,
developing optogenetic approaches employing highly red-
shifted light has proven to be technically challenging.
In this work, we have developed an upconversion nano-
material-based system to act as a mediator for facilitating opto-
genetic control using near-infrared (NIR) light (980 nm).
Upconversion nanoparticles (UCNPs) have attracted significant
biomedical interest due to their inherent ability to emit high-
energy visible light upon excitation with low-energy NIR
light.
10
UCNPs possess a number of favorable characteristics
compared to conventional fluorescent materials including
high resistance to photoblinking and photobleaching, long
luminescence lifetimes and high signal-to-noise ratio due to
weak background autofluorescence.
11
Moreover, by carefully
controlling the experimental conditions during the synthesis
process, the physicochemical properties of UCNPs (e.g. size,
emission spectra, surface chemistry) can be finely-tuned based
on the application of interest.
12
So far, these features have
made UCNPs particularly ideal for numerous bio-applications
including cellular labeling, imaging and drug delivery.
13–15
By
generating a neural–material interface using UCNP-embedded
polymer hybrid scaffolds, we provide one of the first demon-
strations of upconversion nanomaterials as NIR-excitable
internal light sources to achieve optogenetic control of
mammalian neuronal activity (Fig. 1).
Acquiring optogenetic control of neurons relies on the
exogenous expression of light-sensitive ion channels within
the neuronal plasma membrane.
16
Channelrhodopsin-2
(ChR2) is one example of a microbial ion channel that con-
ducts cation influx upon blue light illumination
(∼470–475 nm).
17
The stable expression of ChR2 in mamma-
lian neurons can thus render the neurons sensitive to acti-
vation with blue light.
2
Given that the blue light-responsive
ChR2 is well-established and widely-used in optogenetics, we
sought to design UCNPs that can activate ChR2 upon NIR illu-
mination. The upconversion process (from NIR light to visible
light) in UCNPs is permitted by the doping of trivalent lantha-
nide ions within a host matrix.
10
Among other synthetic vari-
ables, selecting the proper lanthanide dopants is a critical
factor that determines the wavelength-specific luminescence
spectrum of the UCNP.
18
To this end, a combination of the
lanthanide ions Yb
3+
(a sensitizer, which has an absorption
cross-section in the NIR spectral region) and Tm
3+
(an activa-
tor, which has optical emission in the blue spectral region) is
reported to be a suitable pair to achieve blue emission
(∼475 nm).
19,20
We selected to employ Yb
3+
/Tm
3+
-doping for
our study, since the resulting upconverted blue emission over-
laps with the excitation spectra of ChR2,
21
thus suggesting the
possibility for UCNP-mediated optogenetic activation.
We sought to synthesize monodispersed UCNPs that exhibit
strong upconversion luminescence under NIR illumination. To
this end, we synthesized β-hexagonal-phase UCNPs consisting
of a sodium yttrium fluoride (NaYF
4
) host matrix co-doped
with Yb
3+
/Tm
3+
using a co-precipitation method from a pre-
vious report.
22
With modifications to the experimental con-
ditions and further optimization, we synthesized NaYF
4
:Yb
3+
/
Tm
3+
UCNPs with 30 mol% Yb
3+
and 0.2 mol% Tm
3+
. Trans-
mission electron microscopy (TEM) shows a spherical mor-
phology of the particles, with an average diameter of 36.2 ±
1.5 nm (Fig. 2a). In order to preserve the optical integrity of
the nanoparticles, we further coated the nanoparticles with an
Fig. 1 Schematic diagram depicting the generation and application of polymer–UCNP hybrid scaffolds for optogenetic neuronal activation. Up-
conversion nanoparticles (UCNPs) were embedded within polymeric films to form biocompatible hybrid scaffolds for neuronal culture. The UCNPs
served as internally excitable light sources that convert NIR light into blue light, thus facilitating optogenetic activation of channelrhodopsin (ChR)-
expressing neurons.
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inert thin-shell of NaYF
4
. This is reported to be an important
step because lanthanide dopants exposed on the nanoparticle
surface can undergo deactivation due to surface defects, lattice
strains and interactions with the surrounding solvents.
23,24
An
inert shell can thus minimize surface quenching-induced
emission loss and preserve high UCNP luminescence.
25
The
epitaxial growth of a NaYF
4
shell was carried out on the UCNP
cores as reported previously.
24
TEM confirmed the shell for-
mation and the monodisperity of the resulting core–shell
UCNPs, which displayed an average diameter of 46.7 ± 2.4 nm
(Fig. 2a). The core–shell UCNPs exhibited an absorption peak
at 980 nm (Fig. S1 in ESI†) and peak emission in the blue spec-
tral region at ∼475 nm upon excitation with 980 nm NIR light
(Fig. 2b and c). Powder X-ray diffraction (PXRD) patterns
exhibit peak positions and intensities that can be indexed with
the β-hexagonal phase NaYF
4
crystal structure,
26
further con-
firming the high crystallinity of the UCNPs (Fig. 2d). The
quantum yield of our core–shell UCNPs is expected to be
∼1–3%, based on previous reports.
27,28
With the as-synthesized core–shell UCNPs exhibiting favor-
able spectral properties for ChR2 activation, we next encapsu-
lated the particles within a biomaterial scaffold. Biomaterials
are becoming increasingly useful for neural engineering since
they can be tailored to interact with nervous tissue on a mole-
cular, cellular and tissue level.
29
At the same time, biomater-
ials can serve as reliable scaffolds to load and immobilize
cells, biomolecules and other materials over long periods of
time. To this end, compared to a solution-based delivery of
UCNPs that can suffer from diffusion of particles over time
and high variability in particle distribution, we elected to
embed the UCNPs within a biomaterial scaffold for our
studies. We used poly(lactic-co-glycolic acid) (PLGA) as the
underlying biomaterial scaffold, since it is biocompatible, bio-
degradable, FDA-approved and widely-used for in vivo trans-
plantation in the nervous system.
30,31
In order to acquire a
uniform nanoparticle distribution within the polymer matrix,
UCNPs from a stock solution were first mixed with the PLGA
solution. After thorough mixing to disperse the UCNPs, the
Fig. 2 Monodisperse core–shell UCNPs with blue emission upon near-infrared excitation were synthesized. (a) Transmission electron microscopy
(TEM) image of core NaYF
4
:Yb
3+
/Tm
3+
(30/0.2 mol%) nanoparticles (left, scale bar: 100 nm) and core@shell NaYF
4
:Yb
3+
/Tm
3+
(30/0.2 mol%)@NaYF
4
UCNPs (right, scale bar: 50 nm). (b) Photographic image of a cuvette with a suspension of UCNPs under laser excitation at 980 nm. (c) Upconversion
emission spectrum of core–shell UCNPs in hexane solution (1 mg mL
−1
) under near-infrared light excitation at 980 nm. (d) Powder X-ray diffraction
(PXRD) patterns of the core–shell UCNPs, showing all peaks to be well-indexed in accordance with the β-hexagonal-phase NaYF
4
crystal structure
(Joint Committee on Powder Diffraction Standards file no. 16-0334).
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PLGA–UCNP solution was spin-coated to form thin films on
circular glass coverslips (Fig. 3a). A number of parameters were
optimized including the amount of UCNP loading, percentage
of PLGA and speed of spin-coating (see ESI†for details). Flex-
ible thin films (∼0.5 µm thickness) of PLGA–UCNP were gene-
rated that could be peeled from the glass coverslip (Fig. 3b).
Moreover, the hybrid scaffolds showed blue luminescence
under NIR irradiation, owing to the distribution of UCNPs
throughout the polymer film (Fig. 3b). This luminescence
spectra of the hybrid scaffold was evaluated using fluorescence
emission spectroscopy (Fig. S2 in ESI†), which confirmed the
optical integrity and presence of UCNPs within the polymer
films. Increasing the power of NIR illumination was further
observed to exponentially increase blue light emission from
the scaffolds, exhibiting a linear relationship on a double log-
arithmic scale (Fig. S3 in ESI†). Further varying the initial
amount of UCNPs mixed with the PLGA solution (prior to
spin-coating) enabled the generation of films with varying con-
centrations of embedded UCNPs (quantification shown in
Table S1†), which were characterized using scanning electron
microscopy (SEM) (Fig. S4 in ESI†) and atomic force
microscopy (AFM) (Fig. S5 in ESI†). By tuning both the concen-
tration of UCNPs embedded within the film and the power of
NIR illumination, a range of blue emission output can be
achieved (Fig. S6 in ESI†).
The polymer–UCNP hybrid scaffolds were then utilized to
achieve optical activation of cultured neurons expressing
ChR2. Mouse hippocampal neurons were isolated from post-
natal day 0 (P0) mice pups and plated on the hybrid scaffold
surface pre-coated with Matrigel. Thereafter, the neuronal cul-
tures were transduced with ChR2-tdTomato via lentiviruses
after 4–5daysin vitro (DIV) growth. The neurons attached and
showed complex morphology with extensive dendritic branch-
ing by 14 DIV (Fig. S7 in ESI†). Cell viability assay further con-
firmed that the hybrid scaffold did not have any adverse
effects on neuronal survival (Fig. S8 in ESI†). SEM images
display neurons cultured on the polymer scaffold with uni-
formly distributed UCNPs, thus allowing high localization of
UCNPs in close cellular proximity (Fig. 3c).
At 14–15 DIV, we conducted whole-cell patch clamping
experiments to measure the light-mediated neuronal response.
The whole-cell patch clamp technique is a method to record
the ionic current flow across the neuronal membrane, as well
as changes in the membrane potential, in response to an
applied stimuli (e.g. light).
32
Since ChR2 is a light-gated cation
channel, the opening of this channel upon light illumination
Fig. 3 Blue-emitting UCNPs were embedded within thin polymer films of PLGA and cultured with hippocampal neurons. (a) Schematic depicting
preparation of poly(lactic-co-glycolic acid) (PLGA)-embedded UCNP hybrid scaffolds for hippocampal neuronal cultures. (b) Photographic image of
the flexible hybrid scaffold film. The blue luminescence under 980 nm laser excitation illustrates the encapsulation of UCNPs throughout the PLGA
film. Scanning electron microscopy (SEM) image shows the distribution of the UCNPs within the PLGA film. Scale bar: 500 nm (bottom), 100 nm
(inset). (c) Scanning electron microscopy (SEM) image of a hippocampal neuron (pseudo-colored red for contrast) cultured on the polymer–UCNP
hybrid scaffolds at 14 DIV. Scale bar: 5 µm (left), 200 nm (right).
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would facilitate the influx of cations and lead to an inward
current flow to drive neuronal depolarization.
2,17
The mem-
brane-localized expression of ChR2 was first confirmed by the
presence of tdTomato fluorescence using confocal microscopy
(Fig. 4a). Not surprisingly, the inward current response could
be induced in these cells when light-emitting diode (LED)-
generated 470 nm light was supplied via the microscope objec-
tives (Fig. 4b). We then delivered 980 nm NIR light using an
external optical cable setup (Fig. S9 in ESI†). Remarkably, an
inward current response was immediately evident upon NIR
light illumination, similar as that observed by using the con-
ventional 470 nm light (Fig. 4b).
While the presence of an inward current flow is an indi-
cator of successful UCNP-mediated optogenetic control, the
generation of nerve impulses (i.e. action potentials) would
provide stronger and more reliable functional evidence for our
approach. Nerve impulses mediate the information flow in the
nervous system, and can only be generated by a sufficient
increase (i.e. depolarization) in the membrane potential past a
predefined threshold.
33
Given that the time course of up-
conversion in UCNPs is less than half a millisecond (Fig. S10
in ESI†), we were able to apply low NIR pulse widths that are
generally used for optogenetic studies. In our studies, we
checked for nerve impulse generation by applying brief pulses
(3 ms pulse duration) of light to neurons under current-clamp
recording mode. Remarkably, nerve impulses elicited with
980 nm NIR illumination were comparable to those elicited
with 470 nm light (Fig. 4c), indicating a sufficient blue lumine-
scence from the UCNPs to activate ChR2 and drive neuronal
depolarization beyond the nerve impulse firing threshold.
Moreover, 980 nm illumination of the ChR2-expressing
neurons cultured on the hybrid scaffolds enabled the gene-
ration of time-locked, sustained naturalistic trains of impulses
with millisecond-timescale temporal resolution, in response to
light pulses delivered at 1 Hz, 5 Hz and 10 Hz (Fig. 4d).
Impulses were generated with frequencies up to 20 Hz using
NIR light (Fig. S11 in ESI†). This is in contrast to 980 nm illu-
mination of control substrates, i.e. ChR2-infected neurons on
PLGA substrates (Fig. S12 in ESI†) and non-infected neurons
on PLGA–UCNP substrates (Fig. S13 in ESI†), which showed
negligible membrane depolarization and a lack of nerve
impulse generation. This indicates the inability of 980 nm NIR
light alone to activate ChR2-expressing neurons. The mem-
brane depolarization and the reliable generation of action
Fig. 4 Channelrhodopsin-expressing neurons grown on polymer–UCNP films exhibit robust excitatory activation upon NIR-light illumination, com-
parable to blue LED illumination. (a) Hippocampal neurons expressing ChR2-tdTomato (red). Scale bar: 50 µm. (b) Representative traces of inward
current flow in voltage-clamped neuron evoked by 470 nm light (5 mW) and 980 nm light (1 W). (c) Magnified action potential induced by 470 nm
light (top) and 980 nm light (bottom). (d) Representative traces showing repetitive action potentials in a current-clamped hippocampal neuron
evoked by 1 Hz, 5 Hz and 10 Hz train of light pulses from 470 nm light (top; 250 mW, 3 ms pulse width) and 980 nm light (bottom; 1 W, 3 ms pulse
width).
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potentials described above was obtained for polymer films
containing a minimal UCNP concentration of 8.3 µg mm
−2
,
wherein lower concentrations did not generate action
potentials.
The physicochemical properties of the upconversion nano-
material can play a significant role in determining the overall
effectiveness of our approach. For instance, we had initially
considered employing upconversion nanomaterials in the
form of nanorods, due to closer resemblance to bulk
materials.
18
These upconversion nanorods (UCNRs) were syn-
thesized as reported previously,
18
using the Yb
3+
/Tm
3+
dopants
to achieve a blue luminescence spectra similar to the core–
shell UCNPs shown in Fig. 2 (Fig. S14 in ESI†). These rod-
shaped particles, with a length of about 200 nm and width of
50 nm, were embedded within PLGA films and cultured with
neurons (Fig. S15 in ESI†). However, the whole-cell patch
clamping experiments showed inconsistent and unreliable
nerve impulse generation, especially at higher frequency
pulses (Fig. S16 in ESI†). In addition, nerve impulse generation
with the UCNR-based hybrid scaffolds required higher NIR
power (2 W) and longer NIR pulse durations (50 ms) compared
to the core–shell UCNP-based hybrid scaffolds (1 W, 3 ms). In
fact, core–shell UCNP-based hybrid scaffolds enabled success-
ful generation of action potentials at longer NIR pulse duration
as well, in the form of sustained spikelets (Fig. S17 in ESI†).
The ineffectiveness of UCNRs can be attributed to multiple
factors, including non-uniformity and random orientation
after film formation (Fig. S18 in ESI†), as well as surface-
quenching effects from the lack of an inert shell (complete
shell formation on UCNRs is a technical challenge due to
highly anisotropic particle growth
34
). Besides, the larger size
and rod-like morphology of UCNRs makes them less favorable
when considering future applications for in vivo studies.
35
In
this way, the design and systematic testing of nanomaterials is
invaluable for evaluating their prospects in neural applications
such as optogenetics.
In summary, we have developed a novel and promising
proof-of-concept/method as a complementary technique to
expand the optogenetic toolbox. Our approach is unique in
that we employ UCNPs as mediators to convert NIR light into
opsin-activating visible light. From an application perspective,
UCNPs can be excited with relatively inexpensive continuous
wave (CW) diode lasers; this is in stark contrast to two-photon
approaches, which require high-energy femtosecond pulsed
lasers and instrumentation for raster scanning of a focused
beam over the region of interest.
36
Moreover, while conventional
approaches focus on engineering new opsin-variants to impart
red-shifted spectral sensitivity,
37,38
our approach entailed syn-
thetically-tuning UCNPs to emit at a specific wavelength of
light (e.g. 475 nm blue light) upon NIR excitation to activate
ChR2. In this way, our current in vitro approach can be useful
to assess the UCNP-mediated stimulation of a variety of
different opsins, which exhibit peak sensitivity to different
wavelengths of light (e.g. green, yellow) and can be excitatory
or inhibitory.
9
In other words, a library of combinatorial
UCNPs can be systematically designed and easily tested for sti-
mulating a corresponding library of well-established opsins.
This offers an additional wavelength for optogenetic control,
which can facilitate multiplexed stimulation and spatio-
temporally confine emission to regions containing UCNPs.
Such a level of control can help explore specific biological
questions at the single-cell level on these unique neural-
material interfaces, which are not only biocompatible but also
provide a biologically-relevant functionality. With rising inter-
est in improving control over neuronal activity and function,
creating effective interfaces between neurons and external
materials will be invaluable. Overall, the integration of chemi-
cal control and nanotechnology with optogenetics holds great
potential for advancing applications in neuroscience research.
Acknowledgements
K.-B. Lee acknowledges financial support from the National
Institute of Health (NIH) Director’s New Innovator Award
[1DP20D006462-01], National Institute of Neurological Dis-
orders and Stroke (NINDS) of the NIH [1R21NS085569-02],
N. J. Commission on Spinal Cord grant [CSR13ERG005],
National Science Foundation (NSF) [NSF CHE-1429062] and
the Rutgers IAMDN. Z. P. Pang acknowledges financial support
from National Institute on Drug Abuse (NIDA) R21 DA035594,
Sinsheimer Scholar Award and Robert Wood Johnson Foun-
dation. We would like to thank Benjamin Deibert for help with
PXRD, Dr. Tae-Hyung Kim for help with AFM and Hudifah
Rabie for help with UCNP synthesis.
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