muscle contraction is due to cyclic interactions between 
 myosin ii 
 motor proteins and actin filaments powered by the cellular fuel adenosine-5-triphosphate 
 ( atp ) . in in vitro assays one uses surface adsorbed 
 myosin motors , or preferentially myosin motor fragments such as heavy 
 meromyosin ( hmm ) , to propel fluorescent actin filaments .
whereas such 
 assays have inspired a range of nanotechnological applications , actin and myosin have not previously been interfaced with vertical 
 nanowire arrays .
we chose gap as our material system because 
 of the established biocompatibility of gap nanowires and the generally advantageous optical properties of the 
 iii  v semiconductor materials .
nanowires were produced by gold - particle - seeded metal 
 organic vapor phase epitaxy ( movpe ) ( figures 1a and s1 ) . because nanowire 
 growth is initialized by gap assembly at the interface of the substrate 
 to the gold particle , the wire size and array geometry ( figures 1a and 2a )
can be defined 
 by electron beam lithography ( ebl)-defined gold patterning . the use 
 of regular nanowire spacing enables the growth of arrays with uniform 
 wire length . after testing several surface derivatization procedures 
 ( see supporting information , results section ) 
 with respect to actomyosin motility quality ( speed and fraction of 
 mobile actin filaments ) we chose atomic layer deposition of al2o3 on nanowires ( figure 1a ) and surrounding areas , creating suitable surface contact angles 
 for motility without the need for prior 
 silanization .
together with the gap nanowire core diameter ( 3050 
 nm in different arrays ) , the 60 nm of al2o3 coating 
 resulted in a total nanowire diameter of 150171 nm ( 160 
 nm unless otherwise stated ) .
the wire length was either 0.96  
 0.03 or 5  0.2 m ( mean  standard deviation ) in 
 different arrays . in vitro motility studies
were performed by infusing hmm ( 120 g / ml ) into flow 
 cells formed by a chip with nanowire arrays ( figure 1 ) as the ceiling and a coverslip as the floor .
irrespective 
 of the actual geometry ,  top  and  bottom  
 in the text below are defined as indicated in figure 1b .
the hmm - induced sliding of alexa fluor 488 phalloidin labeled 
 actin filaments was observed after infusion of assay solution ( 1 mm 
 mgatp ; 2225 c ) .
we used a nikon eclipse te300 inverted microscope with 
 hg - lamp illumination , nikon 100  1.4 na oil immersion objective , 
 fitc filter set [ excitation , 465495 nm ; dichroic mirror cutoff , 
 505 nm ; emission barrier filter , 515555 nm ] and a hammamatsu 
 emccd camera .
further details of the experimental methods are given 
 in the supporting information , methods 
 section , e.g. , quantitative analysis of the motility quality of filaments 
 moving up and down wires ( supporting figures s2s3 ) .
principle of experiments . a , nanowires are grown from gold seed 
 particles ( i ) by metal organic vapor phase epitaxy ( ii ) and coated 
 with al2o3 by atomic layer deposition ( iii ) . 

b , schematic illustration of fluorescently labeled actin filaments 
 ( red ) that are propelled by hmm motors adsorbed to the al2o3 surface of nanowires . note , illustration not to scale .
nanowire arrays . a , scanning electron microscopy 
 ( sem ) image of 
 nanowires in 50  50 m array with 1 m 
 interwire distance and 5 m nanowire length .
b , fluorescence micrograph from in vitro motility assay experiment 
 with the same type of array geometry as in panel a before the addition 
 of atp .
actin filaments 
 ( bright fluorescent lines ) are aligned with nanowire arrays being 
 attached to hmm on the tip of the nanowires .
strongly fluorescent 
 spots on nanowires attributed to light - guiding from fluorescent actin 
 filaments interacting with the wires along their length .
occasional 
 fluorescent spots outside the array in panel b ( arrows ) are attributed 
 to actin filaments attached to nanowires spuriously formed by parasitic 
 growth ( examples of such wires also indicated in panel a ) and are 
 therefore in focus .
we found that actin filaments 
 can be bound to and propelled by 
 hmm adsorbed to al2o3-coated gap nanowires . 

first , we infused fluorescent actin filaments in the absence of atp 
 where hmm is expected to bind the filaments but not propel them . under 
 these conditions , filaments attached themselves horizontally across 
 the top ( figure 1b ) of a nanowire array , apparently 
 binding to multiple nanowire tips ( figure 2 ) . in addition , several strongly fluorescent spots were associated 
 with individual wires in sparse ( interwire distance 1 m ; figure 2 ) arrays and along the edges in denser arrays ( interwire 
 distance < 300 nm ; not shown ) .
we attribute these spots to filaments 
 that are attached vertically along individual wires ( see below ) . upon 
 addition of atp , we observed actin motility on areas surrounding the 
 nanowire arrays ( figure 3a ) as well as on top 
 of arrays having interwire distance of 1 m ( figure 3b ; supporting information , movie 1 ) and < 300 nm ( figure 3c ; supporting information , movie 2 ) .
the filaments 
 on the top moved from tip to tip , and , ultimately , some of them were 
 propelled vertically down the wires ( figure 3d ) .
occasionally , filaments were also observed climbing up wires 
 from the surrounding surface ( figure 3e ) .
however , 
 observation of the latter phenomenon with 5 m long nanowires 
 required refocusing of the microscope because the underlying substrate 
 and the top of the nanowire array were located in different focal 
 planes ( figure 3f ) .
this fact was important 
 as it also enabled us to unambiguously identify the plane in which 
 filaments were moving ( see also supporting information , figure s4 and movie 3 ) .
a , motility on al2o3 surface outside 
 autofluorescent array illustrated by maximum pixel intensities over 
 50 consecutive images at 2.5 s frame rate .
b , the maximum 
 pixel intensity in sequence of 220 images at 2.5 s frame rate for array with 1 m interwire distance and 5 m 
 nanowire length .
arrows show starting points for some trajectories 
 of actin filaments sliding on top of the array .
c , image sequence 
 of array with circular and square nanowire patterns connected by zigzag 
 lines with nanowires .
scanning electron 
 micrograph in inset showing area on another array similar to that 
 in circle in main figure ) .
d , e , time sequence of filaments climbing 
 down and up a nanowire , respectively . for dashed rectangles in panel 
 d , see figure 4c .
the crosses in panel e indicate 
 tracking of the trailing filament end to estimate the length of the 
 filament segment that is vertically attached along the nanowire ( fluorescent 
 spot of increasing intensity ) at each given point in time .
f , fluorescence 
 images where focus changed from the bottom surface ( left ) to the top 
 of the wires ( right ) . see also supporting information movie 3 and figure s4 for similar effects of change in focus on 
 another area . a climbing event , whether 
 up or down a wire , was associated with 
 an initial rapid increase in intensity of a fluorescent spot centered 
 on the wire ( figure 3 ; supporting information , movie 1 ) , followed by rapid decline 
 in fluorescence of this spot ( see further below ) .
such intensity variations 
 (  blinking  ) were not observed in the absence of atp , 
 and they were seen in all cases where the filament transition from 
 horizontal to vertical movement could be directly observed .
by observing individual 
 actin filaments first approach and then climb up or down individual 
 nanowires , we could identify and quantify light guiding of fluorescent 
 emission from actin filaments specifically bound to nanowires . on 
 a sparse array ( nanowire length 5 m , interwire distance 1 m ) , 
 filaments that climbed up or down wires were visible from the top 
 as strongly fluorescent spots ( figure 3b ; also 
 figure 2b , figure s2 and supporting information , movie 1 ) with a time - dependent intensity 
 proportional to the fraction of the filament length attached vertically 
 along the wire via hmm . importantly
the number of fluorophores along 
 the wires , at each given point in time , is directly proportional to 
 the filament length along the wire in a well characterized way .
this 
 follows because there is precisely one binding site for the aph fluorophore 
 per actin monomer .
further , the distance between 
 the monomers is well - known giving 362 
 aph binding sites per m ( see further supporting 
 information , methods ) .
thus , taking advantage of this homogeneous 
 fluorescence labeling , we were able to estimate , with good accuracy , 
 the number of fluorophores on the filament segments along the wire 
 by tracking the trailing end of individual filaments whose leading 
 end progressed up or down the wire ( figure 3e ) .
results of such tracking for three filaments climbing 5 m 
 long nanowires are illustrated in figure 4a , together with intensity 
 measurements that show progressive linear increase in intensity ( itip ) emitted from the nanowire tip as the filament 
 length along the wire increases .
further , this intensity saturated 
 at low values ( figure 4b ) for nanowires with 
 length of less than 1 m ( supporting information , figure s5 ) , whereas the maximum intensity itip for filaments moving along 5 m long nanowires increased 
 in proportion to the maximum intensity ( ih ; proportional to length ) measured when the filaments moved horizontally . 
 the results for the two wire lengths show that only the part of the 
 filament held close to the wire surface contributes significantly 
 to the fluorescence signal .
more importantly , however , the results 
 in figure 4a , b for 5 m long wires can not 
 be explained by , for example , the projection of fluorescence of vertically 
 aligned filaments because the depth of focus of our objective ( < 0.6 
 m ; figure 3f ; supporting 
 information , figure s4 and movie 3 ) , was much too limited to 
 collect light from all along the nanowire .
instead , light - collection 
 and light - guiding by the nanowire is necessary to integrate the filament 
 fluorescence intensity into a nearly diffraction limited spot , in 
 the focal plane of the wire tip .
this view is further supported by 
 the effects of changes in focus on the intensity ( supporting information , figure s4 ) measured within a 0.5  
 0.5 m roi centered on a nanowire .
a , average background - subtracted pixel fluorescence intensity from 
 the tip of nanowires ( 0.5  0.5 m region of 
 interest ( roi ) ; 16 bit images ; intensity units , iu16 ) plotted 
 against the distance that three different filaments ( different symbols ) 
 have moved down a wire of 5 m length .
the nonzero intercept 
 on the vertical axis is due to the tip of horizontally moving filaments 
 entering the roi without movement of the trailing filament end that 
 is tracked .
this may occur for filaments moving on top of a sparse 
 nanowire array as the number of strongly attached actin propelling 
 heads on the wire tips vary with time .
thus , the leading filament 
 end would temporarily move faster than the tracked trailing end due 
 to buckling caused by previous faster sliding of the trailing end . 

b , total background - subtracted fluorescence 
 intensity ( itip ; average intensity  
 number of pixels ) observed from nanowire tip due to nanowire - climbing 
 filaments plotted against the total intensity ( ih ) of the same filaments when sliding on top of the array ( for 
 5 m high nanowires ; filled circles ) or on the surface between 
 nanowires ( 0.96 m wires ; open squares ) .
measurements using 
 matlab after conversion to 8 bit images corresponding to units , iu8 .
c , time course of the fluorescence signal at three different 
 locations as indicated in figure 3d , illustrating 
 the potential to use the light guiding effect for fluorescence detection 
 with increased signal / noise ratio . measurement using imagej and 16 
 bit images showing average intensity per pixel . for details of imagej 
 and matlab - based analyses ,
interestingly , the behavior 
 was different in denser nanowire arrays 
 ( < 300 nm spacing ) where similar apparent light - guide effects were 
 mainly observed at edges ( indicated by strong fluorescent spots in 
 figure 3c ; see also supporting 
 information , movie 2 ) .
quantitatively , we estimate that 
 50% of the light emitted 
 from fluorescent actin filaments attached vertically along wires in 
 sparse ( 1 m interwire distance ) arrays is emitted from their 
 tip .
we draw this conclusion , first , from the measured slope near 
 unity ( 1.04 ; 95% ci [ 0.91 , 1.17 ] ) of the observed linear relationship 
 ( r  0.91 ) between itip and ih for nanowires of 5 m 
 length ( figure 4b ) .
second , we reasonably assume 
 that less than 50% of the light from a filament ( moving in an optically 
 homogeneous environment across the nanowire tips ) is available for 
 collection by the objective .
we further note that our experimental 
 setup does not fulfill any geometrical conditions for intensity enhancement 
 ( neither of itip nor ih ) by fluorescence interference contrast effects .
our results are thus consistent with the notion 
 that the entire emitted fluorescence intensity from a fluorescence 
 labeled actin filament , held by hmm along a nanowire in a sparse array , 
 couples into the wire and that 50% of the light intensity is emitted 
 at the wire tip , whereas 50% enters the surface substrate . in this 
 context
it is important to note that zincblende gap has a direct band 
 gap of 2.78 ev , corresponding to 450 nm light wavelength .
this should 
 be compared to excitation wavelengths in the range 465495 
 nm and emission in the range 500600 nm ( observed in range 
 515555 nm with the current microscope filter set ) .
therefore , 
 the gap nanowires and the underlying gap substrate should absorb and 
 reflect light only to a minor degree .
our observations of efficient 
 coupling of light into the gap nanowires 
 and light guiding along these wires are consistent with previous findings 
 using other semiconductor materials . in those prior studies ,
photonic 
 confinement was shown for a range of nanowires ( e.g. , zno , cds ,
their high refractive index and the surrounding dielectric material 
 allow for wave - guiding with amplified output .
furthermore , enhanced 
 light trapping and emission has been observed using light coupled 
 into tips of iii  v semiconductor nanowires that are grown in 
 ordered arrays and coated with a dielectric material with a smaller 
 refractive index than the core ( e.g. , al2o3 ) .
the degree of enhancement appeared to depend 
 on fine - tuning of the dimensions of the iii 
this is consistent 
 with our observations of different degrees of light guiding for different 
 interwire distances and suggests that there is appreciable room for 
 optimization of light collection and emission in future experiments .
our demonstration of 
 light - guiding of gap nanowires in a biological buffer has a range 
 of interesting applications in biosensing ( see also ref ( 5 ) ) .
thus , the nanowires would 
 perform well as optical sensors that capture analyte molecules via 
 recognition elements ( e.g. , antibodies or oligonucleotides ) immobilized 
 on their large surface areas .
if fluorescent secondary recognition 
 molecules are sandwiched via the analytes to the nanowires , light 
 intensity proportional to the number of such recognition molecules 
 would be emitted from the wire tip . in the following , we illustrate 
 and quantify the advantages of this approach using a specific example 
 where the velocity and fraction of motile actin filaments are evaluated . 

high - throughput assays with this capability are of considerable interest 
 per se , e.g. , to evaluate effects of drugs on actomyosin motility .
the efficient detection of actin filaments is also important in motor - driven diagnostics and biocomputation devices .
the 
 achievable s / n ratio for the detection of actin filament speed using 
 nanowire arrays is much higher than that achievable on flat substrates 
 and automated read - out is greatly simplified ( see below ) .
we quantify 
 the fraction of motile filaments by the ratio of the numbers of time - varying 
 and time - invariant fluorescent spots ( figure s2 , supporting information ) and determine the filament velocity 
 from the rate of change of the fluorescence intensity ( figure 4a ; see also figure s3 , supporting 
 information ) .
the fraction of motile filaments and velocity 
 for nanowire - climbing actin filaments determined in this way was 81 
  3.8% ( mean  standard error of the mean ; se ; n = 136 filaments ) and 2.18  0.38 m / s ( n = 10 filaments ) , respectively , compared to 84  3.9% ( n = 83 ) and 5.16  0.15 m / s ( n = 22 ) , respectively , for a standard trimethylchlorosilane ( tmcs)-derivatized flat surface . crucially , however , the time - varying 
 fluorescence intensity for a filament moving down a nanowire was detected 
 with considerably higher s / n ratio than for a horizontally moving 
 filament ( figure 4c ) .
to exemplify , for two 
 different filaments the s / n ratio was 13 and 17 , respectively , when 
 moving down a wire ( using a region of interest [ roi ] of 0.5  
 0.5 m ; figure 4c ) compared 
 to less than 4 when the filaments moved on top of the nanowire array 
 ( whether using small , 0.5  0.5 m or large 
 rectangular rois ; figure 3d ) .
qualitatively 
 similar data were observed for several more filaments and also for 
 filaments that first moved on the flat underlying substrate and then 
 climbed up wires . under the present , far from optimized conditions 
 ( see below ) , the 
 limit of detection ( lod ) was 50100 fluorophores on individual 
 wires as suggested by observation of the photobleaching of stationary 
 actin filaments attached via hmm to the wires , assuming 362 fluorophores 
 per m of an actin filament ( see above and supporting information , methods ) .
here , the lod was defined 
 as the number of fluorophores that could be detected with s / n ratio 
 of 3 or greater .
the number of 50100 fluorophores per wire 
 corresponds to the amount of protein accumulating on the surface of 
 a nanowire with 100 nm diameter in less than 1 h at 1 pm solution 
 concentration . however , importantly the 
 lod is probably somewhat lower than 50100 fluorophores as 
 there is most likely less than 362 monomers per m of the filament , 
 e.g. , due to prior photobleaching events .
averaging the signal 
 over a large number of wires ( nwire ) would 
 increase the s / n ratio in proportion to ( nwire ) .
for instance , a 100-fold 
 increase in s / n ratio would be achieved by averaging over 10  000 
 nanowires , which could be readily fitted into < 100  100 m area .
this would allow detection of analyte at 1 pm concentration 
 in minutes or 10 fm concentration in an hour . unlike in the present study where actin filaments were bound to 
 only a small fraction of the nanowires , recognition elements and analytes 
 in biosensor applications
the averaging should therefore 
 be readily automated , e.g. , by appropriately adjusting array interwire 
 distance , objective magnification , and ccd pixel size to each other .
several additional improvements would markedly increase the s / n 
 ratio and further lower the lod .
this includes oblique illumination 
 of the observation area together with reduced flow - cell height for 
 reduced out - of - focus background and use of laser illumination instead 
 of an hg lamp to increase excitation intensity at 480 nm wavelength . 

furthermore , a major source of noise , absent in a general biosensor 
 application , is attributed to fluorescent actin filaments floating 
 in solution .
these unbound model analytes were not eliminated by rinsing 
 as would be implemented for any other analyte , following specific 
 binding to recognition molecules on the wires .
finally , it is likely 
 that a theoretical analysis of the underlying optical mechanisms would 
 allow optimization of the in- and out - coupling of light , further enhancing 
 s / n ratio .
thus , the fluorescence wavelength , the nanowire material , 
 length , diameter , spacing , and the thickness of the oxide shell might 
 affect coupling of light into the wire .
this coupling may also depend 
 on the distance of the fluorophore to the nanowire surface ( 40 nm 
 for hmm - actin ) , the position of the emitting 
 fluorophores along the wire , and the polarization 
 of fluorescence .
a second 
 key result of our study is the unique method to unambiguously demonstrate 
 specific analyte  detector interaction using molecular motor 
 propelled cytoskeletal filaments .
this was enabled by ( i ) measurement 
 of the length of overlap between a fluorescence labeled filament and 
 the long axis of the nanowire , ( ii ) a well - defined number of fluorophores 
 per m of the filament , and ( iii ) molecular - motor driven variation 
 of the filament  nanowire overlap .
here we used heavy meromyosin 
 propelled actin filaments labeled with a maximum of one fluorophore 
 per monomer ( 362 monomers/m ) via the strong phalloidin  actin interaction .
however , other 
 motor systems , such as the kinesin - microtubule system , may also be 
 used , enabling wide applicability due to different dependence of motor 
 function on surface chemistry ( cf . ref ( 36 ) ) for the two systems .
to the best of our 
 knowledge , the present results also constitute the first demonstration 
 of actomyosin motility on al2o3 substrate and 
 on vertical nanowire arrays , opening for a range of opportunities . 

first , the observed climbing of filaments along nanowires enables 
 three - dimensional motor driven lab - on - a - chip devices .
second , the 
 observation of actin filament propulsion across the tips of 160 nm 
 wide nanowires at 1 m interwire spacing opens for unique studies 
 of actomyosin function .
for instance , it will allow testing of hypotheses 
 about the motility of actin filaments over a sparse layer of motors with minimal interference from surface 
they 
 are also of interest for studying contractile events at low myosin 
 densities on actin filaments as occurs physiologically at low activation 
 levels in muscle cells when only a fraction of the regulatory units 
 along the filament are turned on to allow myosin binding . for developments 
 of functional assays
it may be beneficial to make the vertical parts 
 of the nanowires resistant to hmm binding in analogy to a previous 
 study where kinesin propelled microtubules were observed moving between 
 tips of micrometer wide poly(dimethylsiloxane ) ( pdms ) pillars .
we have demonstrated 
 light guiding with 
 high efficiency along al2o3 coated gap nanowires 
 of appreciable relevance for biosensing applications . with
further 
 improved s / n ratio as discussed , sensor elements based on individual 
 vertical nanowires could achieve fm  pm sensitivity , and arrays 
 of such nanowires could perform appreciably better .
the use of molecular 
 motor driven propulsion of fluorescent filaments along the nanowires 
 to detect specific molecular interactions may have broad applicability 
 in the evaluation of biosensor performance .
in addition , our findings 
 lay the foundations for novel types of fundamental studies and nanotechnological 
 applications of molecular motors .
 semiconductor nanowire arrays offer 
 significant potential for biosensing 
 applications with optical read - out due to their high surface area 
 and due to the unique optical properties of one - dimensional materials . 
 
 a challenge for optical read - out of analyte - binding to the nanowires 
 is the need to efficiently collect and detect light from a three - dimensional 
 volume . 
 here we show that light from fluorophores attached along several 
 m long vertical al2o3 coated gallium 
 phosphide nanowires couples into the wires , is guided along them and 
 emitted at the tip . 
 this enables effective collection of light emitted 
 by fluorescent analytes located at different focal planes along the 
 nanowire . 
 we unequivocally demonstrate the light - guiding effect using 
 a novel method whereby the changes in emitted fluorescence intensity 
 are observed when fluorescent cytoskeletal filaments are propelled 
 by molecular motors along the wires . 
 the findings are discussed in 
 relation to nanobiosensor developments , other nanotechnological applications , 
 and fundamental studies of motor function . 
Actomyosin Motility Interfaced with Nanowire
Arrays
Light-Guiding by Nanowires
Applications of Light-Guiding
New Method for Evaluating Biosensor Performance
Further Implications
of Results
Conclusions
Supplementary Material