Gate manipulation of ionic conductance in a nanochannel with overlapped electric double layers

Sensors and Actuators B 215 (2015) 266–271

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Gate manipulation of ionic conductance in a nanochannel with overlapped electric double layers Li-Hsien Yeh a,∗,1 , Yu Ma b,1 , Song Xue c , Shizhi Qian d,∗ a

Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China Mechflutech LLC, Severna Park, MD 21146, USA d Institute of Micro/Nanotechnology, Old Dominion University, Norfolk, VA 23529, USA b c

a r t i c l e

i n f o

Article history: Received 10 February 2015 Received in revised form 23 March 2015 Accepted 26 March 2015 Available online 3 April 2015 Keywords: Nanofluidics Zeta potential Stern layer Charge regulation Field effect transistor

a b s t r a c t To improve the development of gated nanofluidic devices for emerging applications, analytical expressions are derived to investigate the gate manipulation of surface charge property and ionic conductance in a pH-regulated nanochannel with overlapped electric double layers (EDLs). Results show that the EDL overlap effect is relatively significant at low pH and salt concentration when a negative gate potential is applied. If pH is low, the EDL overlap effect on the field control of zeta potential of the nanochannel is remarkable at large positive gate voltage, while that effect on ionic conductance is significant at large negative gate voltage. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The ability to control the ion transport, which is highly dependent on surface charge properties and ion concentration distributions [1,2], in nanofluidics such as nanochannels and nanopore plays a key role for emerging applications such as energy harvesting [3,4], rectification of ion current [5–7], and sensing and analyzing of (bio)nanoparticles [8–10]. To achieve active control, nanofluidic field effect transistors (FETs) [11,12], referring to gate electrode-embedded nanofluidic devices, have been developed recently. Many experimental results demonstrated that both the surface charge properties [13] and, accordingly, the transport of ions, fluids, and biomolecules [14–23] in the nanochannel can be actively controlled by tuning the gate voltage imposed on the gate electrode. Thus far, a majority of researches on the numerical modelings of the FET control in nanofluidics have been performed [23–35], while analytical studies [36–40], which are more useful for experimentalists, are still very limited. This is because some limited assumptions, such as assuming a fixed surface charge density on the

∗ Corresponding authors. Fax: +886 5 5312071. E-mail addresses: [email protected] (L.-H. Yeh), [email protected] (S. Qian). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.snb.2015.03.053 0925-4005/© 2015 Elsevier B.V. All rights reserved.

dielectric channel wall [36], and neglecting the presence of protons and Stern layer effect [36,37], are typically adopted to derive analytical solutions. Moreover, the existing analytical models assumed the electric double layers (EDLs) of the nanochannel are not overlapped [36–40]. This implies that the ionic concentrations at the center of the nanochannel are assumed to be their bulk value, and the potential at the channel center arising from the charged channel wall vanishes. Although ignoring the EDL overlap effect on the transport phenomena in gated channels makes mathematical models much simpler, it is obviously no longer applicable to the modern FET-gated nanofluidic devices, the characteristic sizes of which are commonly smaller than 30 nm [15–23]. In this study, we analyze the gate manipulation of surface charge property and ionic conductance in a pH-regulated nanochannel with significantly overlapped EDLs under various solution conditions. Analytical expressions are derived for the first time to take into account the effects of EDL overlap, Stern layer, electroosmotic flow (EOF), surface site dissociation/association reactions on the dielectric channel wall, and the presence of multiple ionic species. Note that this is the first attempt to develop an analytical model for the gated nanofluidic system with overlapped EDLs. The developed model is capable of predicting the zeta potential, surface charge density, EOF velocity, and ionic conductance in a gated nanoslit at any levels of pH and slat concentration used in experiments.

L.-H. Yeh et al. / Sensors and Actuators B 215 (2015) 266–271

267

 

respectively; z = zi  = 1 and Ct0 = C10 + C20 = C30 + C40 = Cb + 10−(pH−3) for pH ≤ 7 and Cb + 10−(14−pH−3) for pH > 7. Note that because H+ and OH− ions are taken into account, the present model is applicable to any levels of pH in nanofluidic experiments. The boundary conditions associated with Eqs. (1)–(3) can be expressed as h + d, 2

(4)

h , 2

(5a)

dϕ d h − ε0 εf = −s at x = , 2 dx dx

(5b)

ϕ = Vg at x =

ϕ =  = s at x = ε0 εd = Fig. 1. Schematic of the gate manipulation of the surface potential (s ), zeta potential (d ) and ionic current/conductance in a pH-regulated nanochannel filled with electrolyte solution containing K+ , H+ , Cl− , and OH− . Vg is the gate potential applied to the gate electrode, c the central potential in the nanochannel, and d the thickness of the dielectric channel layer.

2. Mathematical model As schematically depicted in Fig. 1, we consider a nanofluidic FET, including two gate electrodes embedded outside the dielectric layers of the nanochannel walls. The nanochannel of height h, length l, width w, and dielectric layer thickness d, connects two large reservoirs and is filled with an electrolyte solution containing ionic species of valence zi and concentration Ci , i = 1, 2, . . ., N. The Cartesian coordinates, x and z, are adopted with the origin located at the center of the nanochannel. A uniform electric field, Ez = V/l, with V being the potential bias applied in the z-direction to drive the transport of ions (ionic current) and fluid (EOF). The surface charge property and, accordingly, the electrokinetic ion and fluid transport in the nanochannel can be manipulated by actively regulating the gate potential Vg imposed on the gate electrodes. In nanofluidic experiments, the ionic strength of the electrolyte solution is typically adjusted by a background salt KCl (or NaCl) and its pH by HCl and KOH (or NaOH). Therefore, we consider four major ionic species (i.e., N = 4), namely, K+ (or Na+ ), H+ , Cl− , and OH− . Letting Ci0 (i = 1, 2, 3, and 4) be the bulk concentrations of these ions, respectively, and Cb the background salt concentration of KCl (or NaCl), we obtain C10 = Cb , C20 = 10−(pH−3) , C30 = Cb + 10−(pH−3) − 10−(14−pH−3) , and C40 = 10−(14−pH−3) for pH ≤ 7; C10 = Cb − 10−(pH−3) + 10−(14−pH−3) and C30 = Cb for pH > 7 [41,42]. Assuming that both l and w are remarkably larger than h, and ions confined inside the Stern layer of thickness ds are immobile [1], without considering the ion concentration polarization effect [43,44] the electric potential arising from the charged channel wall can be described by d2 ϕ =0 dx2 d2  =0 dx2

h



h

 h

h
− ds < x <

2

= c and



 0
h − ds 2

 (3)

In the above, ϕ, , and are the electric potentials within the dielectric channel layer, Stern layer, and liquid, respectively; ε0 and εf are the permittivity of vacuum and the relative permittivity of

4

Fz C exp (−zi F /RT ) electrolyte solution, respectively; e = i=1 i i0 is the mobile space charge density; R, F, and T are the universal gas constant, Faraday constant, and absolute fluid temperature,

(7)

In the above, εd is the relative permittivity of the dielectric channel layer;  s , s , and d are the surface charge density, surface potential, and zeta potential of the nanochannel, respectively; c is the electrical potential at the center of the nanochannel (central potential). Note that if the EDL overlap effect is neglected, c vanishes (i.e., c = 0), which is often assumed for nanochannels in the previous literatures for simplicity [36–39,45]. Many experimental results show that the wall of dielectric channels (e.g., SiO2 , Al2 O3 , Six Ny ) in contact with aqueous solution reveals a charge regulation behavior. This implies that  s depends substantially on the solution properties such as pH and ionic strength. To account for this effect, we assume that the following two major surface reactions, AOH ↔ AO− + H+ and AOH+ ↔ 2 AOH + H+ , occur on the dielectric channel wall. The surface charge density of the nanochannel can be expressed as [46]



s = (eNt × 10 )



2

[H+ ]s − Ka1 Ka2

18

2

[H+ ]s + [H+ ]s Ka2 + Ka1 Ka2

(8)

,

where e is the elementary charge; Nt (in sites/nm2 ) is the total number site density of functional groups, including AOH, AO− , and AOH+ , on the dielectric channel surface; Ka1 and Ka2 are the equi2 librium constants for the aforementioned dissociation reactions of AOH and AOH+ , respectively; [H+ ]s = 10−3 × C20 exp (−Fs /RT ) is 2 the molar concentration of H+ at the dielectric channel surface. If we let the surface capacitance of the Stern layer s = ε0 εf /ds , based on the solutions to Eqs. (1) and (2) the interface boundary conditions Eqs. (5b) and (6b) yield the following two equations (detailed derivations can be found in the supplementary data)



ε0 εd

Vg − s d



− s (s − d )



(2) zF − RT

(6b)

d = 0 at x = 0 dx

= −(eNt × 1018 )



(6a)

d d h = ε0 εf at x = − ds 2 dx dx

(1)

,

d2 e 2FCt0 =− =− sinh ε0 εf ε0 εf dx2

ε0 εf

h − ds , 2

= d at x =

s (s − d ) = sign (d )





2

[H+ ]s − Ka1 Ka2 2

[H+ ]s + [H+ ]s Ka2 + Ka1 Ka2

 4RTε0 εf Ct0 cosh



F RT

 − cosh

,

(9)

 F  c

RT

.

(10)

The exact solution to Eq. (3) subject to Eqs. (6a) and (7) is [47] = c +

 2RT ln [CD( l m)], F

(11)

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L.-H. Yeh et al. / Sensors and Actuators B 215 (2015) 266–271

where CD(l|m) is a Jacobian elliptic function of argument l = (x)/[2 exp (Fc /2RT )] and m = exp (2Fc /RT ). Here −1 = D =  ε0 εf RT/2F 2 Ct0 is the Debye length. Substituting Eq. (6a) into Eq. (11) and assuming ds → 0, which is valid since the thickness of the Stern layer is generally very thin (ca. 0.3 nm) compared to the channel height, one gets the following relationship between d and c , d = c +





CD l x →

h 2

   m

-8

10

10

(12)

-10

10

By simultaneously solving Eqs. (9), (10) and (12) using Matlab Fsolve function with adjustable parameters (Nt , Ka1 , Ka2 , and s ) for charge-regulated dielectric channels, s , d , and c can be obtained. Typical values of Nt , pKa1 , pKa2 and s for the dielectric channels made of silica are 3.8–8 sites/nm2 , 6–8, −3–0, and 0.15–2.9 F/m2 , respectively [40]. Here pKa1 = − log Ka1 and pKa2 = − log Ka2 . Subject to the boundary conditions, (i) uz = 0 at x = h/2 − ds and (ii) duz /dx = 0 at x = 0, the fully developed EOF velocity profile can be described as [39] uz =

ε0 εf Ez

− d ).

(

  2W   V

8Wε0 εf RTCt0

l

 Gc =

2WF 2 RTl

(e uz )dx +

0

= Gv + Gc Gv =





Ez F 2 RT

 4  i=1





(h/2−ds )

cosh 0



(D1 C10 + D2 C20 )

F RT

(h/2−ds )

 

 −





Di Ci0 exp

h dx − cosh 2 F RT

 F  c

RT

1

10

2

10

3

10

(14)

,

(15)



To validate that our model is capable of capturing the underlying physics of the ionic conductance in a nanochannel, it is used to predict the experimental data available from the literature. Fig. 2 depicts the ionic conductance (G) in a silica nanochannel of h = 18.7 nm, l = 60 ␮m, and w = 12.5 ␮m as a function of the background salt concentration of KCl, Cb , in the range of 10−3 –1000 mM. Symbols represent the experimental data of Cheng and Guo [48] at pH = 5.8, and solid line represents the result of the present analytical model. For comparison, the analytical result proposed by Ma et al. [39] without considering the EDL overlap effect (dashed line) is also included in Fig. 2. Because the background salt is KCl, the corresponding ionic diffusivities of K+ (D1 ), H+ (D2 ), Cl− (D3 ), and OH− (D4 ) are 1.96, 9.31, 2.03, and 5.30 (×10−9 m2 /s), respectively [49]. Fig. 2 shows that the result of the present model with the EDL overlap effect (solid line) successfully describes the general trend of the experimental data of Cheng and Guo [48]; however, that of Ma et al. [39] without considering the EDL overlap effect (dashed line) fails in the regime of low salt concentration under which the EDLs are significantly overlapped. The results from both models are the same when the EDLs are not overlapped at relatively high salt concentration. The difference between the two models increases at low salt concentration, which is expected because the EDL overlapping is significant for the considered channel height of 18.7 nm.

0

10

Cb (mM)

dx

dx + (D3 C30 + D4 C40 )

3. Results and discussion

-1

10





zF − i RT



where I is the ionic current through the nanochannel; Di is the diffusivity of the ith ionic species; Gv and Gc are the ion conductance contributed from the convective current due to EOF and the electromigrative current arising from the imposed axial electric field.

-2

10

To clearly demonstrate the EDL overlap effect on the FET control in a nanochannel under various solution properties, the fitted parameters of Nt = 7 sites/nm2 , pKa1 = 6, pKa2 = −2, and s = 0.15 F/m2 are used in the following discussions. For compari-

0



exp 0

(h/2−ds )

-3

10

Fig. 2. Nanochannel conductance (G) versus the background salt concentration Cb . Symbols represent the experimental data of Cheng and Guo [48] at h = 18.7 nm, l = 60 ␮m, w = 12.5 ␮m, and pH = 5.8. Solid and dashed lines represent, respectively, the results with and without considering the EDL overlap effect at Nt = 7 sites/nm2 , pKa1 = 6, pKa2 = −2, and s = 0.15 F/m2 .

(13)

(h/2−ds )

w/ EDL overlap w/o EDL overlap

-11

10

Once d and c are obtained, the ion conductance of the nanochannel can be evaluated by (see the detailed derivation in the supplementary data), I = G= V

-9

G (S)

2RT ln F

-7

10

(h/2−ds )





exp 0

F RT



dx

(16)

son, both results with (lines) and without (symbols) the EDL overlap effect inside the nanochannel are all presented. For illustration, we consider a gated nanochannel of fixed geometry at h = 10 nm, w = 10 ␮m, l = 10 ␮m, and d = 30 nm. 3.1. Influence of solution pH Fig. 3 depicts the influence of pH on the gate manipulation of the central potential c , zeta potential d , and ionic conductance G, in the nanochannel at the background KCl concentration Cb = 0.1 mM with the corresponding Debye length D ≤ 30 nm. Under this background salt concentration, the EDL overlapping inside the nanochannel is significant, as shown in Fig. 3a where c is not zero. The central potential depends on pH and the gate potential, Vg . Note that if d < 0, taking into account the EDL overlap effect results in more counterions, namely, H+ and K+ , confined inside the nanochannel and, therefore, a higher surface proton concentration [H+ ]s . The latter leads to a larger negative d (Fig. 3b), while the former yields a larger nanochannel conductance (Fig. 3c). Consistent with the experimental observations of FET control in nanofluidics [13,14], Fig. 3b shows that d can be regulated from negative to positive at low pH and sufficiently high positive Vg . Note that if pH increases, the nanochannel’s surface charge increases accordingly, resulting in more counterions confined near the channel surface and, therefore, a larger Vg required to tune the sign of d . Fig. 3b also reveals that the EDL overlap effect on d is relatively insignificant at the following two cases: (i) high pH regardless of

L.-H. Yeh et al. / Sensors and Actuators B 215 (2015) 266–271

269

-8

G (S)

10

-9

10

Vg=- 20 V Vg=- 10 V Vg= 0 V -10

10

3

5

7

pH

9

11

Fig. 4. Nanochannel conductance G as a function of pH for various Vg at Cb = 0.1 mM. Lines and symbols represent the results with and without considering the EDL overlap effect, respectively.

potentials Vg . If the FET is floating (i.e., Vg = 0 V), the EDL overlap effect on G is insignificant when pH is sufficiently high and extremely low. The former arises from the higher the solution pH the larger the nanochannel’s surface charges, thus attracting more counterions inside the nanochannel and, therefore, leading to a thinner apparent EDL thickness. The latter can be attributed to the higher ionic strength of solution at apparently low pH. On the other hand, if the FET is active (i.e., Vg = −10 and −20 V), the EDL overlap effect on G is insignificant at high pH, but still significant at low pH. Note that a decrease in pH results in not only a decrease in the EDL thickness, which makes the EDL overlap effect insignificant, but also an increase in the concentration of protons. It is known that the H+ ions have larger ionic diffusivity than the other cations K+ . If the FET is applied at a large negative gate potential, the negative zeta potential of the nanochannel significantly increases especially at low pH, as shown in Fig. 3b. This results in a significantly concentrated effect of protons inside the nanochannel, thus remarkably enhancing its conductance. This explains why the EDL overlap effect on the FET control of ionic conductance is nevertheless significant when pH is low. Fig. 3. Central potential c (a), zeta potential d (b), and conductance G (c), of the nanochannel as a function of Vg for various pH at Cb = 0.1 mM. Lines and symbols represent the results with and without considering the EDL overlap effect, respectively.

Vg , and (ii) low pH and apparently large negative Vg . For example, the results with considering the EDL overlap effect yield the relative deviations from those without of ca. 3.6, 1.9, and 1.8% (18.3, 12.3, and 4.5%) for pH = 4, 6, and 8, respectively, at Vg = −20 V (20 V). These behaviors can be explained as a consequence of the following reason. If pH is high and/or the negative Vg is large, the negative d becomes large. An apparently higher concentration of counterions gathered inside the nanochannel, yielding a thinner effective EDL thickness and, therefore, a less remarkable EDL overlap effect. The EDL overlap effect on G shown in Fig. 3c is similar to that of d in Fig. 3b, suggesting that the ionic conductance in a gated nanochannel is primarily dominated by its zeta potential. Compared to the results from the model without considering the EDL overlap effect (symbols), considering the EDL overlap effect (lines) under the considered conditions yields a largest relative deviation of ca. 145% for G at pH = 4 and Vg = −2 V, as depicted in Fig. 3c. This implies that ignoring the EDL overlap effect might result in not only an incorrect estimation in the ionic conductance in the gated nanochannel but also a wrong description of the underlying physics of relevant experimental results using gated nanofluidic devices. Fig. 4 depicts the variations of the ion conductance, G, of the nanochannel as a function of pH for various applied gated

3.2. Influence of background salt concentration For illustration, we fix the solution pH at 3.5 to investigate the influence of the background salt concentration, Cb , because the EDL overlap effect on the FET control in a gated nanochannel is significant at medium low pH, as shown in Figs. 3 and 4. Fig. 5 depicts the influence of Cb on the gate manipulation of the central potential c , zeta potential d , and ion conductance G in the nanochannel. As expected, the lower the salt concentration, the thicker the EDL thickness and, therefore, the more significant the EDL overlapping effect. This yields a more significant deviation of c from zero, as shown in Fig. 5a. Fig. 5b and c also reveals that the FET control efficiency of d and G is more significant at lower Cb because of thicker EDL thickness [39]. It is worth noting that under the considered conditions the EDL overlap effect on the FET control of d (G) is relatively more significant when Vg is positively (negatively) applied at a large value, as shown in Fig. 5b and c. For example, the relative deviations of d (G) with and without the EDL overlap effect are ca. 4.4 and 19.5% (36.9 and 10.3%) at Vg = −20 and 20 V, respectively, when Cb = 0.1 mM. The more significant EDL overlap effect on the FET control of nanochannel conductance at the negatively applied gate voltage and low pH can be attributed to the significantly enhanced concentration of protons as the reasoning employed in Fig. 4. In short, Fig. 5c suggests that for low background salt concentration and small negative gate potential, the EDL overlap effect on the gate manipulation of nanochannel conductance is more important. Under the considered conditions, the

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L.-H. Yeh et al. / Sensors and Actuators B 215 (2015) 266–271

gate manipulation of the nanochannel conductance is significant at low salt concentration. It is interesting to note that the results of G with EDL overlap effect (lines) show a local minimum in the low regime of Cb and this behavior is apparent when the FET is active at a large negative Vg (e.g., −10 and −20 V). The non-linear behavior that G has a local minimum at low salt concentration is consistent with the experimental results reported by Kim et al. [22], Cheng and Guo [48], and Ouyang and Wang [50] for nanochannels with significant EDL overlap effect. This phenomenon can be attributed to an increase in the convective conductance (Gv ) due to the significant EOF, which is dominated by d and e . Apparently, the negative zeta potential and amount of mobile counterions in a nanochannel with EDL overlap effect are larger than those without, which explains why the local minimum of G at Vg = 0 V only appears in the model with EDL overlap effect, as shown in Fig. 6. 4. Conclusions Analytical expressions are derived to investigate the effects of EDL overlapping on the surface charge property and ionic current/conductance in a long gated silica nanochannel with consideration of the Stern layer, electroosmotic flow, surface site dissociation/association reactions, and the presence of H+ and OH− ions. Note that the ion concentration polarization effect in the long nanochannel is neglected in our model. The applicability of our model is verified by comparing its prediction to the existing experimental data of ionic conductance in a nanochannel with significant EDL overlap effect. Effects of the solution pH and background salt concentration on the gate manipulation of zeta potential and ionic conductance in a nanochannel are examined. Results show that the EDL overlap effect is significant at medium low pH, low salt concentration, and slightly negative gate voltage. If pH is low, the EDL overlap effect on the field control of zeta potential (ion conductance) in the nanochannel is significant when a remarkably positive (negative) gate voltage is applied. Neglecting the EDL overlap effect might result in not only an incorrect estimation of ionic conductance over one time but also a wrong description of conductance behaviors. Fig. 5. Central potential c (a), zeta potential d (b), and conductance G (c), of the nanochannel as a function of Vg for various Cb at pH = 3.5. Lines and symbols denote the results with and without considering the EDL overlap effect, respectively.

Acknowledgments This work is supported, in part, by the Ministry of Science and Technology of the Republic of China under Grants 102-2221E-224-052-MY3 and 103-2221-E-224-039-MY3 (L.H.Y.), the State Key Program of National Natural Science of China under Grant No. 51436009 (Y.M.), and NSF CMMI-1265785 (S.Q.).

-7

10

-8

10

G (S)

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2015.03.053.

Vg=-20 V

-9

10

Vg=-10 V Vg= 0 V

-10

10

-3

10

-2

10

-1

10

0

10

Cb (mM)

1

10

2

10

References 3

10

Fig. 6. Nanochannel conductance G as a function of Cb for various Vg at pH = 3.5. Lines and symbols represent the results with and without considering the EDL overlap effect, respectively.

largest relative difference of G with and without the EDL overlap effect is 129.4% at Cb = 0.1 mM and Vg = −2 V. Fig. 6 depicts the variations of the ion conductance, G, of the nanochannel as a function of the background slat concentration Cb for various gate potentials, Vg . Again, the EDL overlap effect on the

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Biographies Li-Hsien Yeh received his Ph.D. in chemical engineering from National Taiwan University (Taiwan) in 2007, and worked as a postdoctoral fellow from 2008 to 2011. Following the postdoctoral research, he spent one year in the Institute of Micro/Nanotechnology at Old Dominion University as a research fellow. In 2012, he joined the Department of Chemical Engineering and Materials Engineering at National Yunlin University of Science and Technology (Taiwan) as an assistant professor. In 2013, Dr. Yeh was awarded the Young Scholar Award from the Taiwan Institute of Chemical Engineers. His current research interests include electrokinetics, microfluidics and nanofluidics, nanopore sensing techniques, and colloid and surface sciences. Yu Ma received B.S. and Ph. D. degrees in Energy Science and Engineering from Harbin Institute of Technology (HIT), Harbin, China, in 2003 and 2008, respectively. Since 2008, he has been a faculty of the School of Energy Science and Engineering at HIT. He has been an Associate Professor since 2013. From 2013 to 2014, he visited Old Dominion University as a visiting scholar. His research interests include micro/nanofluidics, optofluidic device, radiation hydrodynamics, and radiative heat transfer. Song Xue received his Ph.D. in Mechanical and Aerospace Engineering from Old Dominion University in 2014. In 2014, he co-founded the research company, MechFluTech LLC, as project manager in Maryland, USA. His current research interests include electrokinetics, microfluidics and nanofluidics, battery, and energy. Shizhi Qian received his Ph.D. from Mechanical Engineering and Applied Mechanics at the University of Pennsylvania in 2004. He was Assistant Professor in University of Nevada Las Vegas during the period from 2005 to 2008. He moved to Old Dominion University as an Assistant Professor in 2008 and was promoted to Associate Professor in 2011. He has been working in the field of bio-micro/nanofluidics and micro/nanoscale transport phenomena.