the manipulation of electrical surface charge in a rapid and reversible manner is of great importance for a wide variety of biosensing applications , since electrostatic interactions influence the affinity and residence time of biological molecules in the sensing zone .
charge manipulation in nanoscale sensors , such as nanopores , nanochannels and nanotubes , is particularly useful because of the scale of these devices .
solid - state nanopores are a prototypical class of nanoscale sensors , employing electrophoretic forces to thread and translocate charged single nucleic acids or proteins through a nanoscale aperture in an ultrathin membrane
. nanopore surface charge can affect its direct , coulombic interactions with charged biomolecules , and can indirectly influence sensing by generating electro - osmotic flow .
this net flow can modulate molecular capture rate and molecular residence time in the sensing vicinity via hydrodynamic drag .
this effect has previously been characterized using optical tweezers and by varying the nanopore s zeta potential by changing the solution s ph . the ability to modulate the surface charge of a nanopore and , consequently , the associated electro - osmotic flow could allow control of biomolecule translocation speed .
this could in turn lead to the development of advanced single molecule , low - cost characterization techniques for a broad spectrum of clinical samples , including genomic dna , rnas and proteins .
this challenge must be met without compromising the ability of the sensor to probe extremely dilute analytes in a reasonable time frame .
recent efforts towards this goal have primarily involved the fabrication of metallic electrodes at the nanopore surface to induce an electrical field . yet to date , there has been no experimental report of in situ , opto - electrical manipulation of high - density surface charge in nanopores or other nanoscale sensors .
we show here that an off - the - shelf , low - power visible laser beam can be used to directly manipulate the surface charge of a solid - state nanopore .
first , it allows the translocation speed of dna to be tuned without chemically modifying the nanopore surfaces or altering the buffer properties .
second , it can be used to reliably unblock clogged nanopores , keeping them functional over prolonged periods of time .
third , it permits the detection of extremely low molecular weight proteins , which otherwise are nearly invisible to the resistive pulse sensing technique .
a silicon nitride membrane containing a single nanopore was scanned in a custom - made confocal microscope with a tightly - focused green laser ( 532 nm ) , as illustrated in figure 1a .
the continuously recorded ionic current ( i ) flowing through the pore at a fixed voltage reveals an increase when the laser spot overlaps with the nanopore location .
figure 1b illustrates this effect with an intensity surface plot of the ionic current i flowing through this pore as a function of laser spot position . a line scan through
the image reveals a clear symmetric peak in the ionic current , more than double the baseline current level ( fwhm ~500 nm , right panel ) .
this effect is evident in all tested pores ( 4 - 20 nm diameter ) even for laser powers of just a few mw .
although a tightly focused ir laser at high power ( ~1 w ) can produce a small increase in current by locally heating the solution , laser - induced heating can be ruled out here for two reasons : the electromagnetic absorption of water at 532 nm is extremely small , and the laser powers used in our experiments are only a few mw .
a temperature increase of ~35k would be required to produce the observed increase in i , yet heating is estimated to account for a mere 0.005k per mw of focused light based on measurements performed at 800 nm . we thus conclude that direct interaction of light with silicon nitride must cause this increase in the ion current via a photo - conductive effect .
permanent nanopore blocking by small molecules , tiny air bubbles , or other nanoscale particulates is a common limitation for nanopore sensors , severely reducing their functional lifetime . in some cases ,
a series of electrical pulses clears the pore , but often the system must be disassembled and re - cleaned .
we find that in many of these cases a short exposure to a ~5 mw focused laser beam will immediately clear the nanopore .
figure 1c ( left ) shows a raster scan ( bottom left to top right ) of a fully blocked nanopore ( i  0 at v = + 300 or 300 mv ) .
as soon as the laser beam reaches the nanopore location , the pore is cleared .
this is shown chronologically ( figure 1c , right ) as a time trace of i during the laser scan .
we have found that light - induced nanopore unblocking is a highly robust and efficient tool to extend the functional lifetime of solid - state nanopores from a few hours to several days , as detailed in the supporting information ( si ) .
over 1,000 translocation events of 10 kbp dna were acquired in a nanopore ( 5.4 nm ) in the dark ( laser power p=0 ) . at t=150 s
the laser radiation was switched on ( p=2 mw , pre - aligned with nanopore ) as the software continued to record translocation events .
three features are immediately apparent : ( i ) the open pore current ( io ) and the current during translocation ( ib ) both increased when the laser was turned on , ( ii ) the average event amplitude i = io  ib remained at the same level of 0.780.10 na under both conditions , and ( iii ) the mean event dwell - time td increased by roughly a factor of 10 under illumination , as compared to dark conditions .
these features are illustrated in figure 2b , which shows io and i as a function of time throughout the experiment ( top panel ) .
a 150-event running window average over all translocations ( bottom panel ) indeed shows a tenfold increase in the translocation time from ~1 ms to ~10 ms under laser illumination .
we define the retardation factor ( rf ; here rf=10 ) as the increase in mean translocation time under laser light relative to darkness . upon switching off the laser illumination at t=300 s ,
io returned to base level ( 3.6 na ) and the mean translocation dwell time was restored to ~1 ms , suggesting that this effect is completely reversible .
this experiment was repeated for nanopores of various sizes , and in all cases we observed a marked retardation of translocation time under laser illumination ( see si for analysis ) .
previous studies have sensed proteins such as bovine -lactoglobulin or avidin in solid - state nanopores , and antibodies such as anti - biotin fab fragments or anti - biotin in lipid coated nanopores .
however , the detection of small proteins ( mw < 10 kda ) in their native state remains a major challenge , since these proteins translocate through the pore too quickly to produce a resolvable signal given current bandwidth limitations .
we selected ubiquitin ( mw ~ 8.5 kda , 4 nm diameter ) as a representative small protein , relevant to a broad range of biological processes , to demonstrate that the photo - conductive effect can slow small protein translocations and enable their detection and characterization . under dark conditions , purified wild - type ubiquitin ( see si ) translocating through a ~5 nm pore yielded sporadic , brief downward spikes ( red trace , figure 2c ) .
the dwell time of these spikes was estimated to be < 12 s , but neither this nor the current amplitude of ubiquitin translocations could be accurately determined due to bandwidth constraints even at 100 khz , suggesting that the true translocation dwell time of these proteins is shorter than estimated .
laser illumination of the same nanopore at p=4 mw resulted in a marked increase in both the frequency and average dwell time of detected ubiquitin events ( blue trace , figure 2c ) .
the average translocation time increased by at least two orders of magnitude , allowing full characterization of both amplitude and dwell times for ubiquitin translocations .
the increase in event rate under illumination may be attributed to events that were previously undetectable under dark conditions due to bandwidth limitations .
a typical dwell time distribution and fractional blockade current ( ib ) for illuminated ubiquitin translocations are shown in figure 2c ( n > 500 ) .
two prominent timescales are observed for ubiquitin translocation ( 3405 and 89070 s ) , as well as two peaks in the blockade currents ( 0.88 and 0.78 ) , approximated by a sum of two gaussians .
we hypothesize that the two peaks observed in these distributions may be associated with different orientations of the asymmetric  mushroom - like  ubiquitin ( inset ) during translocation .
this hypothesis is consistent with previous reports for other protein / dna complexes . to elucidate the physical origin of light - induced retardation of translocation
, we first characterized the pore current i at varying laser intensity p for a range of nanopore sizes ( 4 - 20 nm diameter , d ) .
figure 3a displays ionic current enhancement ( i(p)/i(0 ) ) , for four representative pores ( d = 5 , 10 , 15 , 20 nm ) . in all cases
linear fits for p<5 mw characterize the response of each pore to light as the initial slope i/p .
a plot of i/p as a function of diameter for 14 pores ( 4 - 20 nm ) exhibits a clear linear dependence on d ( figure 3b ) .
this suggests that when the sinx thickness l and nanopore drilling conditions are maintained , the pores response to light per their surface area ( dl ) is a constant . from figure 3b
the linear dependence of i/p on nanopore diameter suggests that photo - conductance current enhancement is a surface phenomenon rather than a volumetric one ( which would exhibit  d dependence ) .
solid - state nanopores possess weak surface charge , which can dominate pore conductivity at salt concentrations lower than ~100 mm .
however , it has been theorized that high charge densities on nanopore walls can affect electrical conductivity even under high salt conditions .
surface charges induce a diffuse double layer containing an excess of oppositely charged ions , of thickness comparable to the debye screening length . ionic current is enhanced by the electrical double layer in two ways : ( i ) by creating a local imbalance of counter ions within  , which ( ii ) drags water molecules with its motion , creating an electro - osmotic flow ( eof ) which in turn pulls along additional current ( see figure 4a ) .
the total ionic current for a nanopore with a charged surface is : 
 ( 1)i = ibulk+idl+ieof 
 where ibulk is the current due to ions outside the electrical double layer , idl is the current due to ions forming the double layer , and ieof is the current due to net water flow induced by net movement of ions within the double layer .
ieof is proportional to the product of the flow velocity field and the net charge density (r ) , which is only nonzero within . for small  compared to the pore diameter , ( i.e. 0.3 nm at 1 m kcl at 21c ) , only ibulk scales with cross sectional area ( ibulk  d ) whereas idl and ieof scale with circumference ( idl , ieof  d ) .
thus , surface charges can , in principle , explain increased i as well as its linear scaling with d. moreover , if the analyte remains mostly within the bulk pore volume where there is no charge imbalance ,  distance away from the pore walls , we expect i to be independent of surface charge  and thus independent of p. this is consistent with our observations ( figure 2 ) .
previous theoretical work has predicted that surface charge on a nanopore will affect ( i ) translocation times and ( ii ) polymer capture rate .
a negatively charged nanopore produces an eof towards the cathode ( cis chamber , figure 4a ) , increasing drag on a negatively charged translocating polymer , resulting in longer translocation times and reduced polymer capture rates .
our results ( figure 2 ) indicate that the rf for dna translocation grows with laser intensity .
moreover , dna capture rate was reduced by 30% , from 11 s to 8 s , for 2 mw of laser light ( see si ) .
these observations lead us to hypothesize that visible light can induce negative surface charges on silicon nitride , which in turn slows dna translocation through the drag created by eof moving in the opposite direction of dna . to validate this hypothesis
we approximate that for low laser powers the surface charge density (  ) grows linearly with laser intensity , such that  =  p where  is the photo - reactivity of the pore ( c m w ) .
the total ionic current i can be obtained as a function of surface charge density using the following three individual contributions ( see si for full derivation ) : ( i ) ibulk is the electrophoretic movement of each ion species through the nanopore ; ( ii ) idl may be determined from the net charge distribution  for a given wall potential , derived analytically from the poisson - boltzmann equation and from the boltzmann distribution , assuming a cylindrical nanopore and using the debye - hckel approximation ; ( iii ) ieof is a product of  and the eof velocity profile described by the navier - stokes equation .
the grahame equation may be substituted to give  as a function of wall potential . the contributions to the ion current are : 
 ( 2.1)ibulkenkcl(k+cl)d24lv 
 ( 2.2)idl2dklsinh(ep2)v 
 ( 2.3)ieof2d2l2e2[sinh1(ep2)]2v 
 for elementary charge e , permittivity of aqueous solution  , parameter  = 1/kbt ( boltzmann constant kb , absolute temperature t ) , pore length l , applied voltage clamp v , and solution viscosity . the number density of potassium or chloride ions is nkcl , with electrophoretic mobilities k and cl ( see si for numerical values ) .
nanopore diameter d can be determined from tem images , enabling a quantitative estimation of the nanopore photo - reactivity  via eq.2 ( see si figure s7 ) .
we additionally measured the dependence of i on kcl concentration ( 0.01 m to 1 m ) at different p and found it consistent with the model ( see si figure s8 ) .
additional dna translocations were performed using a range of dna lengths , nanopores , and laser powers to determine the role of surface charge in slowing translocation speed .
the results are summarized in figure 4b ( left ) , where rf is plotted against p for five nanopores ( 4.3 , 5.4 , 5.6 , 6.1 , and 7.4 nm diameters ) and four dna lengths ( 0.4 , 3.5 , 5 , and 10 kbp ) .
td was measured for > 1,000 events per laser intensity , and the td histograms were fitted with exponential functions as previously described .
these measurements suggest that rf per mw of light widely varies with pore size and dna length .
2 as described above to obtain  for each of the pores , we then transform the dna rf into surface charge density  using the relationship  =  p.
remarkably , all data points collapse onto a single curve ( figure 4b , right ) .
thus , light - induced slowing is dependent only on the induced surface charge density . controlling the rf value can be achieved either by selecting a highly optically reactive pore ( high  ) under low laser intensity , or using a less optically reactive pore under higher laser intensity .
drilling nanopores in sinx with an electron beam reduces the n / si ratio around the pore due to preferential ablation of n atoms .
moreover , the principal defects in silicon - rich cvd amorphous sinx materials are si dangling bonds , which can trap electrons or holes to become charged . when the n / si ratio approaches 0.8 , the band gap is within visible light energies .
when the n / si ratio of the material surrounding the pore is low enough , we propose that visible light can excite electrons from the ground state across the band gap , trapping them in si dangling bonds .
eventually , trapped electrons would recombine with holes , but a high density of arriving photons could maintain a steady state of negatively charged si dangling bonds , creating a net surface charge density .
we tem - drilled four additional nanopores , all with similar diameters , in a freshly deposited sinx membrane .
two were exposed to the e - beam for 60 s , and two were exposed for 500 s , producing different local n / si stoichiometry ( see si ) . as expected , the pores drilled with a low e - beam dose had relatively low photo - reactivity ( =27 c m w ) , whereas the pores exposed to a high e - beam dose had much higher photo - reactivity ( =70 c m w ) , establishing a direct correlation between e - beam dosage and optical reactivity .
over 1,000 translocation events of 10 kbp dna were acquired in a nanopore ( 5.4 nm ) in the dark ( laser power p=0 ) . at t=150
s the laser radiation was switched on ( p=2 mw , pre - aligned with nanopore ) as the software continued to record translocation events .
three features are immediately apparent : ( i ) the open pore current ( io ) and the current during translocation ( ib ) both increased when the laser was turned on , ( ii ) the average event amplitude i = io  ib remained at the same level of 0.780.10 na under both conditions , and ( iii ) the mean event dwell - time td increased by roughly a factor of 10 under illumination , as compared to dark conditions .
these features are illustrated in figure 2b , which shows io and i as a function of time throughout the experiment ( top panel ) .
a 150-event running window average over all translocations ( bottom panel ) indeed shows a tenfold increase in the translocation time from ~1 ms to ~10 ms under laser illumination .
we define the retardation factor ( rf ; here rf=10 ) as the increase in mean translocation time under laser light relative to darkness . upon switching off the laser illumination at t=300 s ,
io returned to base level ( 3.6 na ) and the mean translocation dwell time was restored to ~1 ms , suggesting that this effect is completely reversible .
this experiment was repeated for nanopores of various sizes , and in all cases we observed a marked retardation of translocation time under laser illumination ( see si for analysis ) .
previous studies have sensed proteins such as bovine -lactoglobulin or avidin in solid - state nanopores , and antibodies such as anti - biotin fab fragments or anti - biotin in lipid coated nanopores .
however , the detection of small proteins ( mw < 10 kda ) in their native state remains a major challenge , since these proteins translocate through the pore too quickly to produce a resolvable signal given current bandwidth limitations .
we selected ubiquitin ( mw ~ 8.5 kda , 4 nm diameter ) as a representative small protein , relevant to a broad range of biological processes , to demonstrate that the photo - conductive effect can slow small protein translocations and enable their detection and characterization . under dark conditions , purified wild - type ubiquitin ( see si ) translocating through a ~5 nm pore yielded sporadic , brief downward spikes ( red trace , figure 2c ) .
the dwell time of these spikes was estimated to be < 12 s , but neither this nor the current amplitude of ubiquitin translocations could be accurately determined due to bandwidth constraints even at 100 khz , suggesting that the true translocation dwell time of these proteins is shorter than estimated .
laser illumination of the same nanopore at p=4 mw resulted in a marked increase in both the frequency and average dwell time of detected ubiquitin events ( blue trace , figure 2c ) .
the average translocation time increased by at least two orders of magnitude , allowing full characterization of both amplitude and dwell times for ubiquitin translocations .
the increase in event rate under illumination may be attributed to events that were previously undetectable under dark conditions due to bandwidth limitations . as with dna translocations ,
this effect was completely reversible . a typical dwell time distribution and fractional blockade current ( ib ) for illuminated ubiquitin translocations are shown in figure 2c ( n > 500 ) .
two prominent timescales are observed for ubiquitin translocation ( 3405 and 89070 s ) , as well as two peaks in the blockade currents ( 0.88 and 0.78 ) , approximated by a sum of two gaussians .
we hypothesize that the two peaks observed in these distributions may be associated with different orientations of the asymmetric  mushroom - like  ubiquitin ( inset ) during translocation .
to elucidate the physical origin of light - induced retardation of translocation , we first characterized the pore current i at varying laser intensity p for a range of nanopore sizes ( 4 - 20 nm diameter , d ) .
figure 3a displays ionic current enhancement ( i(p)/i(0 ) ) , for four representative pores ( d = 5 , 10 , 15 , 20 nm ) . in all cases
linear fits for p<5 mw characterize the response of each pore to light as the initial slope i/p . a plot of i/p as a function of diameter for 14 pores ( 4 - 20 nm ) exhibits a clear linear dependence on d ( figure 3b ) .
this suggests that when the sinx thickness l and nanopore drilling conditions are maintained , the pores response to light per their surface area ( dl ) is a constant . from figure 3b
the linear dependence of i/p on nanopore diameter suggests that photo - conductance current enhancement is a surface phenomenon rather than a volumetric one ( which would exhibit  d dependence ) .
solid - state nanopores possess weak surface charge , which can dominate pore conductivity at salt concentrations lower than ~100 mm .
however , it has been theorized that high charge densities on nanopore walls can affect electrical conductivity even under high salt conditions .
surface charges induce a diffuse double layer containing an excess of oppositely charged ions , of thickness comparable to the debye screening length . ionic current is enhanced by the electrical double layer in two ways : ( i ) by creating a local imbalance of counter ions within  , which ( ii ) drags water molecules with its motion , creating an electro - osmotic flow ( eof ) which in turn pulls along additional current ( see figure 4a ) .
the total ionic current for a nanopore with a charged surface is : 
 ( 1)i = ibulk+idl+ieof 
 where ibulk is the current due to ions outside the electrical double layer , idl is the current due to ions forming the double layer , and ieof is the current due to net water flow induced by net movement of ions within the double layer .
ieof is proportional to the product of the flow velocity field and the net charge density (r ) , which is only nonzero within . for small  compared to the pore diameter , ( i.e. 0.3 nm at 1 m kcl at 21c ) , only ibulk scales with cross sectional area ( ibulk  d ) whereas idl and ieof scale with circumference ( idl , ieof  d ) .
thus , surface charges can , in principle , explain increased i as well as its linear scaling with d. moreover , if the analyte remains mostly within the bulk pore volume where there is no charge imbalance ,  distance away from the pore walls , we expect i to be independent of surface charge  and thus independent of p.
previous theoretical work has predicted that surface charge on a nanopore will affect ( i ) translocation times and ( ii ) polymer capture rate .
a negatively charged nanopore produces an eof towards the cathode ( cis chamber , figure 4a ) , increasing drag on a negatively charged translocating polymer , resulting in longer translocation times and reduced polymer capture rates .
our results ( figure 2 ) indicate that the rf for dna translocation grows with laser intensity .
moreover , dna capture rate was reduced by 30% , from 11 s to 8 s , for 2 mw of laser light ( see si ) .
these observations lead us to hypothesize that visible light can induce negative surface charges on silicon nitride , which in turn slows dna translocation through the drag created by eof moving in the opposite direction of dna . to validate this hypothesis
we approximate that for low laser powers the surface charge density (  ) grows linearly with laser intensity , such that  =  p where  is the photo - reactivity of the pore ( c m w ) .
the total ionic current i can be obtained as a function of surface charge density using the following three individual contributions ( see si for full derivation ) : ( i ) ibulk is the electrophoretic movement of each ion species through the nanopore ; ( ii ) idl may be determined from the net charge distribution  for a given wall potential , derived analytically from the poisson - boltzmann equation and from the boltzmann distribution , assuming a cylindrical nanopore and using the debye - hckel approximation ; ( iii ) ieof is a product of  and the eof velocity profile described by the navier - stokes equation .
the grahame equation may be substituted to give  as a function of wall potential .
the contributions to the ion current are : 
 ( 2.1)ibulkenkcl(k+cl)d24lv 
 ( 2.2)idl2dklsinh(ep2)v 
 ( 2.3)ieof2d2l2e2[sinh1(ep2)]2v 
 for elementary charge e , permittivity of aqueous solution  , parameter  = 1/kbt ( boltzmann constant kb , absolute temperature t ) , pore length l , applied voltage clamp v , and solution viscosity . the number density of potassium or chloride ions is nkcl , with electrophoretic mobilities k and cl ( see si for numerical values ) .
nanopore diameter d can be determined from tem images , enabling a quantitative estimation of the nanopore photo - reactivity  via eq.2 ( see si figure s7 ) .
we additionally measured the dependence of i on kcl concentration ( 0.01 m to 1 m ) at different p and found it consistent with the model ( see si figure s8 ) .
additional dna translocations were performed using a range of dna lengths , nanopores , and laser powers to determine the role of surface charge in slowing translocation speed .
the results are summarized in figure 4b ( left ) , where rf is plotted against p for five nanopores ( 4.3 , 5.4 , 5.6 , 6.1 , and 7.4 nm diameters ) and four dna lengths ( 0.4 , 3.5 , 5 , and 10 kbp ) .
td was measured for > 1,000 events per laser intensity , and the td histograms were fitted with exponential functions as previously described .
these measurements suggest that rf per mw of light widely varies with pore size and dna length .
2 as described above to obtain  for each of the pores , we then transform the dna rf into surface charge density  using the relationship  =  p. remarkably , all data points collapse onto a single curve ( figure 4b , right ) .
thus , light - induced slowing is dependent only on the induced surface charge density . controlling the rf value can be achieved either by selecting a highly optically reactive pore ( high  ) under low laser intensity , or using a less optically reactive pore under higher laser intensity .
drilling nanopores in sinx with an electron beam reduces the n / si ratio around the pore due to preferential ablation of n atoms .
moreover , the principal defects in silicon - rich cvd amorphous sinx materials are si dangling bonds , which can trap electrons or holes to become charged . when the n / si ratio approaches 0.8 , the band gap is within visible light energies .
when the n / si ratio of the material surrounding the pore is low enough , we propose that visible light can excite electrons from the ground state across the band gap , trapping them in si dangling bonds .
eventually , trapped electrons would recombine with holes , but a high density of arriving photons could maintain a steady state of negatively charged si dangling bonds , creating a net surface charge density .
we tem - drilled four additional nanopores , all with similar diameters , in a freshly deposited sinx membrane .
two were exposed to the e - beam for 60 s , and two were exposed for 500 s , producing different local n / si stoichiometry ( see si ) . as expected , the pores drilled with a low e - beam dose had relatively low photo - reactivity ( =27 c m w ) , whereas the pores exposed to a high e - beam dose had much higher photo - reactivity ( =70 c m w ) , establishing a direct correlation between e - beam dosage and optical reactivity .
we have shown that just a few mw of visible laser light focused on a nanopore can induce surface charge densities up to ~1 c m , much larger than the weak native charge of sinx . increased  produces net charge near the pore walls , which , under external voltage , creates an electro - osmotic flow moving in opposition to an anionic translocating molecule .
this flow slows translocating dna and proteins , allowing detection and characterization of small analytes , such as ubiquitin .
previous approaches to slowing analyte translocation have introduced permanent modifications to the nanopore walls , inducing stronger interactions of analytes with the pore surface during translocation . by contrast
, the optoelectronic effect allows completely reversible , in - situ control of  and translocation speed without permanently altering the surface properties of the nanopore or buffer .
moreover , ultra - fast tuning of surface charge by light completely decouples the capture process from subsequent translocation .
solid - state lasers , as well as other light sources , can be readily switched on / off in sub-s timescales , and hence induce current jumps in the nanopores within milliseconds ( see si ) .
the dynamic range of the photoconductive effect is determined by local n / si stoichiometry , which in turn depends on e - beam exposure during nanopore drilling .
we predict that this method will facilitate a broad range of emerging nanopore sensing applications , including genotyping using peptide nucleic acid ( pna ) probes and quantification of epigenetic markers .
ultra - fast and reversible modulation of surface charge is fundamentally important for the development of novel nanosensors . by exploiting the intrinsically high sensitivity of the nanopore ionic current to small modulations of surface charge
, we were able to explore and characterize a new optoelectronic effect , model the increase in electric conductance under visible laser illumination , and propose an underlying mechanism .
we have shown here three useful applications of the technique : ( i ) clearing blocked nanopores and thus significantly extending their useful lifetime ; ( ii ) slowing and tuning the translocation speed of dna ; and ( iii ) detecting and characterizing translocation events of very small proteins , such as ubiquitin .
together , these milestones broaden the range of molecules that can be probed by solid - state nanopores , enhance nanopore sensitivity , and extend the lifetime and throughput of individual pores .
nanopores were drilled on low - stress ( silicon - rich ) , amorphous lpcv - deposited sinx membranes 30 nm thick .
chips were then cleaned using piranha solution and kept in water until mounted on the teflon holder , rendering two independent chambers connected by the nanopore and filled with an electrolytic solution ( 1 m kcl , 20 mm tris , ph 8.0 , 21 c ) .
a pair of ag / agcl electrodes was used to apply an electrostatic potential difference across the chambers separated by the insulating sinx membrane .
the resulting ionic current through the nanopore was recorded using a patch - clamp amplifier ( axopatch 200b , molecular devices cooperation ) .
the whole apparatus was shielded from external electromagnetic noise by a faraday cage . the electrical readout from the amplifier
was filtered with a 10 khz low - pass bessel filter and digitized using a 16 bit a / d card ( national instruments ) controlled by a custom - written labview ( national instruments ) program ( see si for a detailed diagram of the apparatus ) .
the nano - chip was secured to a closed loop nano - positioner ( physik instrumente ) with sub - nm accuracy , also controlled by the labview program .
the nano - positioner was mounted in a custom - built confocal setup , in order to illuminate the nanopore with a focused laser beam .
a 532 nm line from a laser diode ( new focus ) was cleaned by a glan - thompson polarizer ( thorlabs ) , with the final power adjusted on demand by a half - wave plate mounted on a motorized rotating holder .
this resulted in a final dynamic range of 2 w to 17 mw with 500 intermediate values .
the laser beam was expanded to completely fill the back aperture of a 60 , 1.2 na water immersion objective ( olympus uplanapo ) .
the emission side contained a long - pass filter ( chroma hq560lp ) to remove any elastically scattered light , followed by a motorized mirror that acted as a shutter for the apds and allowed direct visualization of the membrane using a ccd camera ( thorlabs dc111 ) .
 nanopores can be used to detect and analyse biomolecules . however , controlling and tuning the translocation speed of molecules through a pore is difficult , limiting the wider application of these sensors . 
 here we show that low - power visible light can be used to control surface charge in solid - state nanopores and can influence the translocation dynamics of dna and proteins . 
 we find that laser light precisely focused at a nanopore can induce reversible negative surface charge densities as high as 1 c / m2 , and that the effect is tuneable on sub - millisecond timescales by adjusting the photon density . by modulating surface charge , we can control the amount of electro - osmotic flow through the nanopore , which affects the speed of translocating biomolecules . 
 in particular , a few mw of green light can reduce the translocation speed of double - stranded dna by more than an order of magnitude and the translocation speed of small globular proteins such as ubiquitin by more than two orders of magnitude . 
 the laser light can also be used to unclog blocked pores . 
 finally , we discuss a mechanism to account for the observed optoelectronic phenomenon . 
Introduction
The photo-conductive phenomenon in solid-state nanopores
Light-induced retardation of DNA and proteins
Light-induced modulation of electro-osmotic flow
Nanopore photo-reactivity depends on 
Conclusions
Methods
Supplementary Material