unless otherwise specified , technical terms here take their usual meanings , specifically those specified in the mcgraw - hill dictionary of scientific and technical terms , 6th edition . the terms “ plasma membrane ” and “ cell membrane ” are used interchangeably throughout to refer to the lipid barrier between the interior and exterior of a eukaryotic cell . the present invention introduces photosensitivity into retinal ganglion cells via the attachment or insertion of fans either across cell membrane or on the outer / inner surface of the cell membrane . fig1 shows one embodiment of a photoresponsive fan made of nanometer - sized semiconductor conjoined with a nanometer - sized metal , which together act as a photodiode . the difference in the chemical potentials of the two components ( the semi - conductor and the metal ) bends the energy bands of the semiconductor near the junction , creating a built - in electrical field . the corresponding schematic band structure of the resulting schottky junction is shown in fig2 . illumination creates electron - hole pairs that separate under the influence of the built - in field , thereby yielding a photovoltage across the structure ( the photovoltaic , or pv , effect ). fig3 shows a schematic diagram of the amphiphilic functionalization of a semiconductor / metal hetero - junction , with the metal and semiconductor segments bearing hydrophilic and hydrophobic coatings , respectively . fig4 schematically illustrates an exemplary approach in accordance with embodiments of the present invention for introducing a fan to a retinal ganglion cell through the use of liposomes . after incorporation of amphiphilic fans into a bilayer membrane of a liposome , assimilation of the liposome into a cell membrane delivers the fans into that membrane , with the hydrophobic portion of each fan immersed in the lipid portion of the membrane , and the hydrophilic extending into the aqueous phase . fig5 schematically shows how an fan localized in a cell membrane , such as that of a ganglion cell , owing to its amphiphilic surface chemistry depolarizes the trans - membrane potential upon absorption of light . excitation of the fan with light generates a photovoltage that counteracts the transmembrane potential , triggering an action potential in the cell . the lower part of the figure shows the equivalent electronic circuit , with notional values of the electrical parameters of the circuit components . fig6 shows a schematic diagram of how a fan embedded into a retinal ganglion cell membrane endows the cell with photosensitivity . light traverses the anterior structures of the eye until impinging on the retina , where the light strikes retinal ganglion cells . incorporation of fans into retinal ganglion cell membranes allows light - induced signals to bypass damaged photoreceptors and generate a ganglion cell action potential triggered by the photovoltaic effect in the fan . fig7 shows a detailed schematic view of the anatomy of the posterior region of the eye , including the ganglion cells connecting to the brain . damaged retinal photoreceptors ( rods and cones ) are connected via bipolar and amacrine cells to the ganglion cells that transmit visual impulses to the brain . fan - mediated intracellular stimulation of the ganglion cells bypasses damaged photoreceptors by triggering an action potential in the ganglion cells . fig8 shows high resolution transmission electron micrographs of two batches of cdse / au nanometer - sized semiconductor / metal hetero - junctions of different sizes ( left 30 nm long ; right 15 nm long ) before micellar encapsulation . the au tip of the rod appears darker than the cdse end owing to its higher electron density . evenly spaced cdse lattice planes show that au growth does not compromise the integrity of the cdse portion . fig9 and fig1 show the birefringence upon application and removal , respectively of external electrical field of cdse rods , and of au / cdse nanometer - sized hetero - junctions with and without light excitation . fig1 is a schematic showing the experimental setup used to probe the photovoltaic effect of fans using atomic force microscopy based surface potential probing ( spp ). fig1 shows the measured surface potential change of fan coated hopg substrate under illumination of 532 nm laser chopped at 10 hz . a surface potential difference of ˜ 15 mv was observed when illumination is turned on and off . this difference may be attributed to the photovoltaic effect of the fans . fig1 shows the optical absorption spectra of cdse / au fans as synthesized ( in toluene ), and in phosphate - buffered saline after functionalization , showing that the optical properties of the fans remain largely unchanged by the derivatization procedure . fig1 schematically shows the cdse / au rods as synthesized , when they are covered by trioctylphosphine oxide and alkylphosphonic acid ( both of which are hydrophobic ). fig1 schematically illustrates that , after surface functionalization with polyethylene glycol , the surface of the au portion of the cdse / au fan is hydrophilic , while the cdse end , still covered by trioctylphosphine oxide and alkyl phosphonic acid , remains hydrophobic . illumination of photovoltaic fans embedded into a nerve cell membrane changes the potential across the nerve cell membrane by 10 mv and thereby triggers the firing of an action potential . us 20030022374 ( hereby incorporated by reference in its entirety ) discloses that illumination of spinach photosystem i particles (˜ 50 nm in size ), delivered to retinoblastoma cells through the use of proteoliposome vesicles , changes the ca 2 + concentration gradient across the plasma membrane of the cells . us 20070028928 ( also incorporated by reference in its entirety ) describes methods of using nanoparticle solar cells to regulate the membrane polarization of excitable biological cells . placement and stabilization of the photovoltaic fan actuators in the membrane mimics the naturally - occurring amphiphilic trans - membrane proteins , which have hydrophobic membrane - spanning domain ( s ) that interact with fatty acyl groups of the membrane phospholipids and hydrophilic domains extending into the aqueous medium on each side of the membrane . nanometer - sized semiconductor / metal hetero - junction fans have different surface chemistry at the metal and semiconductor components . this allows the two to be functionalized separately with hydrophilic and hydrophobic compounds ( fig3 ), thus making them amphiphilic for trans - membrane anchoring . delivery of the nanometer - sized hetero - junction photovoltaic fan to targeted cells and other locations is achieved through the use of liposomes ( fig4 ). amphiphilically - functionalized fans are incorporated into phosphilipid liposomes through the use of standard procedures . when the liposomes are fused with the cells , the nanometer - sized hetero - junction are inserted into the cell plasma membrane . furthermore , functionalizing the liposome with lipid - anchored ligands binds to the specific surface receptors on different types of ganglion cells . appropriate vesicle fusion proteins may be included in the liposomes to effect more efficient fusion with the cell membrane . photovoltaic fan modulation of nerve cell trans - membrane potential , and inducing firing of action potential illumination of photovoltaic fans embedded in nerve cell membranes triggers the firing of the nerve cell action potential . a cell with an embedded semiconductor / metal fan can be approximated electrically by the circuit diagram shown in fig5 ( lower portion ). membranes typically average ˜ 5 nm in thickness and act as a capacitor ( capacitance cm ˜ 1 μf / cm 2 ). membrane ion channels ( mainly resting k + channels ) exert a typical resistivity of r k = 10 kω · cm 2 . the ionic gradient across the membrane , created by atp driven ion - transporters , generates an electromotive potential ( resting potential ) inside the cell 60 mv less than that outside . an embedded semiconductor / metal fan ( with the metal inside the cell ) acts as a photovoltaic cell , which is electrically equivalent to a current source with a resistor and a capacitor in parallel ( fig5 , lower portion ), with the parallel resistor representing the internal resistance within the fan . the electric current varies with the rate of photon absorption . the connection between the fan and the membrane is mediated by the motion of ions in the solution , which is represented by a series resistor between the fan and the membrane . illumination of embedded photovoltaic fans generate a photovoltage that reduces the potential across the ganglion cell membrane by 10 mv . such membrane depolarization causes enough voltage - sensitive na + ion channels to open to generate an action potential that travels down the axon . the density of fans and the light flux to create the 10 mv membrane depolarization can be estimated as follows . a typical fan consists of a 5 nm diameter gold tip grown on a 10 nm diameter cdse nanorod and embedded in the membrane at a density of 10 10 / cm 2 . since the fan occupies only 1 % of the membrane area , its capacitance is much smaller than that of the membrane . such a fan has ˜ 10 nm 2 absorption cross - section for visible light . for every photogenerated electron - hole separation across the plasma membrane ( and assuming no electron - hole recombination , i . e ., r parallel & gt ;& gt ; r k + r serial ), generation of one action potential ( by charging the membrane 10 mv ) every second entails 10 − 8 c / cm 2 charge displacement across the membrane per second . this in turn implies 0 . 02 mw / cm 2 light power density impinging on the membrane of retinal ganglion cells . in typical human eyes only 20 % of the incident light reaches the retina . therefore to achieve 0 . 02 mw / cm 2 power density impinging on the retina , an eye with 4 mm pupil aperture needs to look at a light source of brightness 20 , 000 cd / m 2 . for reference objects in direct sunlight have typical brightness on the order of 50 , 000 cd / m 2 . having generally described this invention , a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified . the examples that follow focus on the use of cdse / au hetero - junctions for the sake of concreteness , but it will be readily appreciated by those skilled in the relevant art that fans can be constructed from a variety of other semiconductor / metal and semiconductor / semiconductor hetero - junctions . for example , fans based upon semiconductor / metal hetero - junctions between group ii - vi , iv , iii - v , iv - vi ( referring to groups of the periodic table ), metal - oxide , or organic semiconductors and a metal , and in particular those based upon si / au , gaas / au , inas / au , and pbs / au hetero - junctions , can be used in the same way as those discussed here . fig8 shows high - resolution transmission electron microscopy images of cdse / au nanometer - sized semiconductor / metal hetero - junctions . both cdse and gold nanocrystals have individually been widely applied in biological labeling in cells , tissues , and animals . integration of both cdse and au into one nanometer - sized composite leads to nanoscale schottky diodes . the cdse / au integrated nanometer - sized composites are synthesized through a two - step procedure . first , cdse nanorods were formed by the reaction of cd and se precursors in a mixture of trioctylphosphine oxide and an alkylphosphonic acid . due to the wurtzite crystallographic structure of the cdse , ( 001 ) and ( 00 1 ) facets of the cdse have enhanced chemical reactivity compared to the { 011 } facets , therefore rod - shaped cdse nanoparticles ( elongated along ( 001 ) direction ) can be formed . second , the cdse rods suspension is treated with a mixture of gold chloride , didodecyldimethyl - ammonium bromide , and hexadecylamine to stabilize the nanocrystals and to reduce the gold chloride to elemental gold . because the two ends (( 001 ) vs . ( 00 1 ) facets ) of the cdse rods differ crystallographically ( and hence chemically ), careful control of growth conditions allows growth of au particles preferentially on one end of each rod . in fig8 the dark end of the rod corresponds to the au tip , which has higher electron density than the cdse end . the particle size (& lt ; 2 nm to & gt ; 100 nm ), aspect ratio ( 1 : 1 to & gt ; 1 : 30 ) or shape ( rod , tetrapod ) can be varied over a wide range by controlling growth conditions . transient electrical birefringence measurements confirm the photovoltaic effect in the cdse / au nanometer - sized semiconductor / metal hetero - junctions . cdse rods exhibit optical birefringence owing to their anisotropy . transient electrical birefringence measurements entail measuring the rise ( anr ) and fall ( anf ) of the birefringence of the cdse rods or cdse / au nanometer - sized heterojunctions ( suspended in a liquid ) in response to the imposition and removal of an external electric field . the rising edge of the birefringence of the rods reflects the rotation of nanometer - sized hetero - junctions to align its dipole preferentially with the external field . the falling edge , the slope of which gives the rotational diffusion constant of the rods , reflects the randomization of the rod orientation on removal of the external field . cdse / au samples of large aspect ratio were used to achieve sufficient transient electrical birefringence signal . fig9 shows the rising and fig1 falling edge of the transient birefringence for three cases : cdse rods before au tip growth , and cdse / au hetero - junctions with and without excitation . excitation was via continuous 532 nm laser at 1 w / cm 2 . heating by the excitation laser was considered and ruled out . compared to cdse rods , the fall and rise of cdse / au birefringence is ˜ 20 times slower . moreover , cdse / au samples in dark and under 532 nm illumination exhibited significantly different transient electrical birefringence . this different rotational response to an external electric field is consistent with photoexcitation redistributing charge in individual cdse / au nanometer - sized semiconductor / metal hetero - junctions , and therefore with a photovoltaic effect . the photovoltaic effect of the cdse / au fan is also independently checked using atomic force microscopy ( afm ) based surface potential probing ( spp ) as schematically shown in fig1 . to prepare the sample for this measurement , a drop of cdse / au fan solution in toluene was put onto a freshly cleaved surface of highly ordered pyrolytic graphite ( hopg ) and placed in an constant electrical field of ˜ 1 × 10 4 v / cm perpendicular to the hopg surface . since the cdse / au fans have built - in electrical dipole , as the fan solution slowly dries ( over ˜ 1 hour ), the hopg surface gets coated with a layer of fans that statistically acquire a preferred orientation along the electrical field perpendicular to the surface . the sample was then installed on a digital instrument multimode afm and an platinum iridium - coated conducting afm tip was used to measure the surface potential ( v surface ) of cdse / au fan coated hopg substrate under light excitation as briefly described below ( refer to fig1 ). the afm tip was lifted a certain height (˜ 60 nm ) above the surface of the sample . a voltage v ( t )= v dc + v ac · sin ( ωt ) was applied on the afm tip ( where ω is the resonant frequency of the tip ) to create an oscillating electrical force between the tip and the surface . the surface potential ( v surface ) was then measured by adjusting v dc to be equal to v surface via a feedback loop so that the force between the tip and surface at frequency ω was zeroed ( hence the oscillation of the tip is minimized ). in our measurements , the fan coated hopg sample was excited by a 532 nm laser chopped at 10 hz . the measured surface potential is shown in fig1 . the difference in the surface potential as the light excitation is turned on and off is around 15 mv . this difference can most likely be attributed to the photovoltaic effect of the cdse / au fans on the hopg surface , since no measurable surface potential change as a function of light excitation was observed on either bare hopg or hopg surface deposited with cdse rods . synthesis of the colloidal fans preferably uses non - polar solvents such as toluene , whereas their use in vivo necessitates ultimately exposing them to water . encapsulation of the fans in phospholipid block - copolymer micelles composed of n - poly ( ethylene glycol ) phosphatidylethanolamine ( peg - pe ) and phosphatidylcholine protected the fans and thereby rendered them compatible with phosphate - buffered saline solution . u . s . pat . no . 7 , 041 , 371 describes methodology for coating the surfaces of semiconductor nanoparticles with polyethylene glycol . fig1 shows the measured optical density of the as - synthesized fan in toluene and then converted to soluble in phosphate - buffered saline for normalized concentration . similar phospholipid micelle encapsulation of cdse - based semiconductor quantum dots for in vitro and in vivo imaging applications rendered them stable in physiological solution and non - toxic to cells up to concentrations of 5 × 10 9 per dots / cell . standard procedures were used to produce cdse nanorods by reaction of cd and se precursors in trioctylphosphine oxide containing an alkylphosphonic acid . in the second step , au precursors were added into the cdse rod solution . the chemical difference of the two ends of the cdse rods allows au particles to be grown on one end preferentially by appropriate control of growth conditions . selenium was dissolved in prepared by dissolving selenium in trioctylphosphine ( se / trioctylphosphine ) 10 % w / w ). cadmium oxide ( 200 mg ), trioctylphosphine oxide ( 3 g ), hexylphosphonic acid ( 0 . 16 g ), and octadecylphosphonic acid ( 0 . 85 g ) were heated to 330 ° c . in a flask under ar on a schlenk line . rapid injection of 0 . 62 g selenium / trioctylphosphine ( 10 % w / w ) at ˜ 330 ° c . into the flask causes fast nucleation of the rods . after 50 min at 300 ° c . to promote further growth of the rods the reaction was stopped , the produced cdse rods dissolved in toluene , precipitated by methanol , and then redissolved in toluene for storage . varying the se pre - cursor injection volume and concentration , the injection temperature , and the molar ratio of the trioctylphosphine oxide ( topo ): hexylphosphonic acid ( hpa ): octadecylphosphonic acid ( odpa ) allows control over the length and width of the rods . typically increased se precursor injection concentration and increased injection temperature leads to formation of larger number of nuclei , therefore the average size of the nanorods decreases . increased molar ratio of phosphonic acids ( hpa and odpa ) to topo leads to formation of nanorods of larger aspect ratio . the au / cdse nanocrystal semiconductor / metal junction was formed by growing au tips on one end of the cdse rods . a stock solution of gold salts was prepared by dissolving aucl 3 ( 1 . 66 mg ), didodecyldimethyl ammonium bromide ( 8 . 33 mg ), and hexyldecylamine ( 15 mg ) in toluene ( 1 g ). to cdse rods ( 20 g , synthesized as described above ) diluted to 10 mg / ml with toluene under ar was added ˜ 1 ml of the stock solution of gold salts at a rate 0 . 1 ml / min . owing to the chemical difference of the two ends of the cdse rods , au particles can be grown on one end preferentially . the thereby - produced au / cdse nanocrystal semiconductor / metal junctions were precipitated with methanol , and then redissolved in anhydrous toluene or chloroform for storage . the as - synthesized au / cdse rods were characterized through use of transmission electron microscopy . the majority (& gt ; 75 %) of au / cdse rods synthesized by the procedure outline above are asymmetric with only a single au tip on the cdse rod . as synthesized , the cdse / au rods are covered by trioctylphosphine oxide and alkylphosphonic acid ( both of which are hydrophobic ), as shown in fig1 . surface functionalization covers the au tips of the cdse / au rods with polyethylene glycol , making them hydrophilic ; the cdse end , still covered by trioctylphosphine oxide and alkyl phosphonic acid , remains hydrophobic ( fig1 ). overall the cdse / au nanojunctions are amphiphilic and compatible with bilipid membranes . cdse / au (˜ 0 . 1 mg / ml ) nanojunctions are suspended in n , n - dimethylformamide containing detergent ( e . g ., 1 % triton x - 100 ) and exposed to polyethylene glycol -( ch 2 ) 10 - sh (˜ 1 mm ) at room temperature for about an hour under stir to coordinate the thiol to the au end . after the reaction , the au / cdse rods are separated from the solution through use of spin - filtration ( filter pore size ˜ 10 kda ). the separated fans are redissolved in phosphate buffered saline containing triton x - 100 . liposomes are prepared sonicating a commercially - available mixture of phosphatidyl choline / cholesterol / phosphatidyl glycerol / phosphatidyl inositol ( in 8 : 10 : 1 : 1 : 2 molar ratio ) in aqueous buffer ( e . g ., 20 mm sodium phosphate , 0 . 15 m nacl , ph 7 . 4 ) at room temperature . amphiphilically functionalized cdse / au fans are inserted into the liposomes through first de - stabilizing the preformed liposomes with a detergent ( e . g ., 1 % triton x - 100 ), followed by addition of amphiphilic cdse / au fans to the destabilized liposome suspension . the cdse / au fan - containing liposomes are then reconstituted by gel - chromatography ( through use of for example sepharose 4b column in 20 mm sodium phosphate , 0 . 15 m nacl , ph 7 . 4 buffer ). the liposome size is characterized through use of dynamic light scattering , while the orientation and density of the fans in the liposome are characterized through use of transmission electron microscopy . selective delivery of liposomes to cells ( e . g . retina ganglion cells ) with specific surface receptors ( e . g . retina ganglion cell specific thy - 1 receptor ), is achieved by conjugating the liposome to the corresponding antibodies by the following procedure , in which the amine terminus of the heavy chain of the antibody is linked to the phosphatidyl glycerol component of the liposome via reductive amination - mediated conjugation technique as detailed in the following two steps . to a suspension of liposomes containing cdse / au rods diluted to total lipid ˜ 5 mg / ml in buffer , and 200 μl of 0 . 6 m aqueous sodium periodate is added for each milliliter of the liposome suspension . after stirring for 30 min at room temperature in the dark the oxidized liposomes are dialyzed against 20 mm sodium borate , 0 . 15 m nacl ph 8 . 4 to remove the unreacted periodate . the liposome suspension is further purified through use of a column of sephadex g - 50 . to 1 ml of a liposome suspension containing 5 mg / ml total lipid is added 0 . 5 ml of a solution of the antibody ( e . g ., anti - thyl ) dissolved in 20 mm sodium borate , 0 . 15 m nacl , ph 8 . 4 , at a concentration of 10 mg / ml . after incubation for 2 h at room temperature is added 10 μl of a 2 m solution of sodium cyanobotohydride in water , and reduction allowed to proceed at 4 ° c . over - night . gel filtration through a column of sephadex g - 50 removes unconjugated protein and excess cyanoborohydride from the liposome suspension . the present invention clearly can be used to introduce photoresponsiveness to a wide variety of cells , and thereby not only to restore lost function but to impart new function to such cells . for example , through use of the present invention photoresponsiveness can be introduced to cells not normally responsive to light , and / or the range of photoresponsiveness can be extended beyond normally visible wavelengths ( to , e . g ., the near infrared ). although the present invention has been described in terms of specific exemplary embodiments and examples , it will be appreciated that the embodiments disclosed herein are for illustrative purposes only and various modifications and alterations might be made by those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims .
the invention relates to imparting photoreactivity to target cells , e . g ., retinal cells , by introducing photoresponsive functional abiotic nanosystems , nanometer - scale semiconductor / metal or semiconductor / semiconductor hetero - junctions that in this case include a photovoltaic effect . the invention further provides methods of making and using fans , where the hetero - junctions bear surface functionalization that localizes them in cell membranes . illumination of these hetero - junctions incorporated in cell membranes generates photovoltages that depolarize the membranes , such as those of nerve cells , in which fans photogenerate action potentials . incorporating fans into the cells of a retina with damaged photoreceptor cells reintroduces photoresponsiveness to the retina , so that light creates action potentials that the brain interprets as sight .