The synergistic effects of stimuli-responsive polymers with nano- structuredsurfaces: wettability and protein adsorption{

Qian Yu,ab Xin Li,a Yanxia Zhang,ab Lin Yuan,*a Tieliang Zhaoab and Hong Chen*a

Received 25th May 2011, Accepted 27th May 2011

DOI: 10.1039/c1ra00201e

Surface modification with stimuli-responsive polymers leads to switchable wettability and

bioadhesion that varies in response to environmental stimuli. The introduction of nanoscale structure

onto surfaces also results in changes to the surface properties. However, the synergistic effects of

stimuli-responsive polymers with nanoscale structures are unclear. In this work, two typical stimuli-

responsive polymers, thermo-responsive poly(N-isopropylacrylamide) (poly(NIPAAm)) and pH-

responsive poly(methacrylic acid) (poly(MAA)), were grafted from initiator-immobilized silicon

nanowire arrays (SiNWAs) with nanoscale topography via surface-initiated atom transfer radical

polymerization. Because of the synergistic effects of the stimuli-responsive conformation transition of

polymer chains and the nano-effects of three-dimensional nanostructured SiNWAs, these new

platforms possess several unique properties. Compared with their corresponding modified flat silicon

surfaces, the introduction of nanoscale roughness enhanced the thermo-responsive wettability of

SiNWAs-poly(NIPAAm) but weakened the pH-responsive wettability of SiNWAs-poly(MAA).

More importantly, these surfaces exhibited special protein-adsorption behavior. The SiNWAs-

poly(NIPAAm) surface showed good non-specific protein resistance regardless of temperature,

suggesting a weakened thermo-responsivity to protein adsorption. The SiNWAs-poly(MAA) surface

showed obvious enhancement of pH-dependent protein adsorption behavior.

Introduction

In recent years, nanomaterials have been widely used for various

biomedical and biotechnology applications, including biosensors,

drug delivery, and diagnostics.1,2 Understanding and controlling

the interactions of nanomaterials with biological molecules, such

as proteins is not only of great theoretical interest but also of

crucial practical importance. Properties inherent to the nano-

material, such as size, curvature and surface chemistry strongly

influence the quality, secondary structure and activity of adsorbed/

conjugated proteins, all of which influence biofunctionality.3–5 On

the other hand, conjugation of nanomaterials with appropriate

proteins offers potential opportunities for biomolecule delivery,

targeted therapy and the detection of small molecules.6–9

Silicon nanowires are widely used as a typical one-dimensional

nanomaterial for field-effect transistors,10 biosensors11,12 and

thermoelectric materials.13,14 Silicon nanowire arrays (SiNWAs)

have attracted increased attention because of their novel

electronic properties and surface activity; they are good

candidate substrates for surface-enhanced Raman scattering15

and solar cells.16 In recent years, some groups have investigated

the biocompatibility of SiNWAs and broadened their applica-

tion to the biomedical and biomaterial fields.17–21 It was found

that the nano-topography of SiNWAs enhanced the interaction

between cells and surfaces and therefore benefits cell adhe-

sion.22,23 With proper modification, SiNWAs acquired unique

wettability that enabled resistance to the adhesion of platelets24

and bacteria.25 Moreover, SiNWAs exhibited efficient tumor cell

capture19 and controlled drug release20 because of their three-

dimensional (3D) nanostructure. However, to the best of our

knowledge, there are few reports concentrating on protein

adsorption on SiNWAs.26

The interfaces that control protein adsorption and desorption

through the modulation of environmental stimuli are important

for many biomedical and biotechnological applications, including

controlled drug delivery, separation science and biosensors.27–31

Persistent efforts have been made towards the design of such smart

interfaces, and many reports concern surface modification with

stimuli-responsive or ‘‘smart’’ polymers.32–35 poly(N-isopropyla-

crylamide) (poly(NIPAAm)) is the best known thermo-responsive

polymer,36 and numerous reports indicate that poly(NIPAAm)-

modified surfaces can switch their wettability, topography and

bioadhesion in response to environmental temperature in the

aCollege of Chemistry, Chemical Engineering and Materials Science,Soochow University, Suzhou, 215123, China. E-mail: chenh@suda.edu.cn;yuanl@suda.edu.cn; Fax: +86-512-65880827; Tel: +86-512-65880827bSchool of Materials Science and Engineering, Wuhan University ofTechnology, Wuhan, 430070, China{ Electronic Supplementary Information (ESI) available: Detailedprocedures of preparation and optimization of SiNWAs, contact angleimages of oil drop and bubble in PBS, and HSA and lysozyme adsorptionon SiNWAs and SiNWAs-poly(NIPAAm) surfaces. See DOI:10.1039/c1ra00201e/

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vicinity of its lower critical solution temperature (LCST).37–40 This

novel property has attracted considerable attention and can be

applied to the fabrication of surfaces with switchable wettability,41

the separation of proteins and other biomolecules,42,43 and the

regulation of the adhesion/detachment of cells,44 platelets45 and

bacteria.46 Poly(methacrylic acid) (poly(MAA)) is another type of

smart polymer that exhibits an extended/collapsed conformational

transition as pH changes; its swelling behavior has been extensively

studied.47–51 When grafted onto a solid surface, this transition

results in pH-responsive changes in surface wettability and charge,

which in turn influences protein-surface interactions because of the

change in hydrophobic and electrostatic interactions.52

Because previous reports have shown that the stimuli-respon-

sivity of surface wettability can be greatly enhanced by nanoscale

roughness,41,53,54 it is natural to explore whether protein adsorp-

tion can also be enhanced when nanoscale topography is

introduced on these smart surfaces. However, the influence of

nanoscale structure on the interactions of smart surfaces with

biomolecules or cells is not consistent. Chen et al. found that the

introduction of nanoscale topography onto poly(NIPAAm)

modified SiNWAs surfaces resulted in the disappearance of

thermo-responsive platelet adhesion.24 In contrast, our recent

research indicates that the poly(MAA)-modified SiNWAs

surfaces exhibit significantly pH-responsive lysozyme adsorption

as compared with poly(MAA)-modified flat silicon surfaces.26

These results suggest that it is important to investigate the

synergistic effects of both stimuli-responsive polymers and

nanoscale structures on ‘‘smart’’ surface properties (wettability

and bioadhesion), which are not only of great theoretical interest

but also of crucial importance for designing and fabricating

novel devices in biomaterial and biomedical applications.

In the present study, two nanoscale surfaces of SiNWAs

modified with typical smart polymers, namely thermo-responsive

poly(NIPAAm) and pH-responsive poly(MAA) were prepared via

surface-initiated atom-transfer radical polymerization (SI-ATRP).

We investigated the synergistic effects of stimuli-responsive

properties and nanoscale roughness on the surface wettability

and protein adsorption. It is interesting that the introduction of

nanostructure results in some novel properties, which may be

favorable for biomaterial and biomedical applications. The

poly(NIPAAm)-modified SiNWAs exhibit high resistance to

non-specific proteins regardless of temperature, whereas the

poly(MAA)-modified SiNWAs show an extremely high capacity

for binding protein at low pH. Most of the adsorbed protein can be

released by simply increasing the pH.

Experimental section

Materials

N-Isopropylacrylamide (NIPAAm, Acros, 99%) was recrystal-

lized from toluene/hexane solution (50%, v/v) and dried under

vacuum prior to use. tert-Butyl methacrylate (tBMA, Aldrich,

98%) was passed through a column of activated basic alumina

prior to use. Copper(I) bromide (CuBr, Fluka) and Copper(II)

bromide (CuBr2, Aldrich) were recrystallized before use.

3-Aminopropyltriethoxysilane (APTES, Aldrich), bromoisobu-

tyryl bromide (BIBB, Fluka) and 1,1,4,7,7-pentamethyldiethyle-

netriamine (PMDETA, Aldrich) were used as received. All other

solvents were purchased from Shanghai Chemical Reagent Co.

and purified according to standard methods before use. Silicon

wafers were purchased from Guangzhou Semiconductor

Materials (Guangzhou, China). The as-received silicon wafers

were cut into square chips about 0.5 cm 6 0.5 cm. Deionized

water was purified by a Millipore water purification system to

give a minimum resistivity of 18.2 MV?cm and used in all

experiments. Fibrinogen (MW = 341 kDa) was purchased from

Calbiochem (La Jolla, CA).

Preparation of polymer-grafted silicon nanowire arrays

The SiNWAs were prepared by chemical etching of crystalline

silicon in HF/AgNO3 aqueous solution (the detailed procedure is

given in ESI{). Then, the initiator was immobilized on the

SiNWAs following the procedures reported previously.40 SI-

ATRP grafting of NIPAAm was carried out in a glovebox

purged with argon. NIPAAm (6.25 g, 55.23 mmol), PMDETA

(0.7 mL, 3.35 mmol) and CuBr (0.16 g, 1.12 mmol) were

dissolved in a 1 : 1 mixture of methanol and water (25 mL). The

reaction solution was sonicated for 2 min and then added to a

glass vessel, into which the initiator-functionalized SiNWAs were

also placed. The polymerization was carried out at room

temperature for 2 h. The obtained SiNWAs-poly(NIPAAm)

surfaces were rinsed with deionized water and dried under an

argon flow. SI-ATRP grafting of tBMA was carried out as

follows. tBMA (4 mL, 25 mmol), CuBr2 (3.3 mg, 0.015 mmol),

CuBr (7.15 mg, 0.05 mmol), PMDETA (0.031 mL, 0.15 mmol)

and 3 mL acetone were mixed in a flask. The heterogeneous

reaction solution was degassed with three freeze–pump–thaw

cycles and then stirred at 60 uC for 20 min until it became clear

and homogeneous. The solution was then transferred to a

Schlenk flask containing the initiator-functionalized SiNWAs via

syringe. The polymerization was allowed to proceed at 60 uC for

4 h. The obtained SiNWAs-poly(tBMA) surfaces were rinsed

thoroughly with acetone, dried under an argon flow, and placed

in a flask containing a mixture of 1,4-dioxane (20 mL) and

concentrated HCl (37%, 3 mL). The reaction was carried out at

80 uC for 2 h to hydrolyze poly(tBMA) to poly(MAA). The

resulting surfaces were thoroughly rinsed with acetone and

ethanol and dried under an argon flow. For comparison,

polymer-grafted smooth silicon surfaces (Si-poly(NIPAAm)

and Si-poly(MAA)) were also prepared following the same

procedures mentioned above.

Surface characterization

The chemical composition of the modified silicon surfaces was

determined with an ESCALAB MK II X-ray photoelectron

spectrometer (XPS) (VG Scientific Ltd.). All XPS data were

analyzed using XPS Peak 4.1 software. The thicknesses of the

polymer grafts on the smooth silicon substrate were measured by

an M-88 spectroscopic ellipsometer (J. A. Woollam Co., Inc.).

The nanostructures and surface morphology of pristine and

modified SiNWAs were observed using scanning electron

microscopy (SEM, S-4800, Japan).

Contact angle measurements

Contact angle (CA) measurements were performed using an SL-

200C optical contact angle meter (Solon Information Technology

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Co., Ltd.) with a heating element placed on the sample stage to

control the temperature of the sample surfaces. The static water

contact angles were measured using the sessile drop method in the

dry state. For poly(MAA)-grafted surfaces, the samples were first

immersed in phosphate buffered saline (PBS) at a specified pH for

30 min and dried immediately in an argon flow. The pH of the PBS

was pre-adjusted by adding aqueous NaOH or HCl solution until

the desired values were reached. To further investigate the

wettability of the surfaces in the wet state, the oil (dichloro-

methane, CH2Cl2) contact angles and captive bubble contact

angles in PBS under different temperatures (for poly(NIPAAm)-

grafted surfaces) or at different pH values (for poly(MAA)-grafted

surfaces) were also measured.

Protein adsorption

Fibrinogen adsorption from PBS solution (1 mg mL21) was

determined by radiolabeling with 125I using a Wizard 3991480

Automatic Gamma Counter (Perkin-Elmer Life Sciences).40 PBS

at different pH levels was used to prepare protein solutions.

Adsorption was allowed to proceed for 3 h under static

conditions. For PNIPAAm-grafted surfaces, the adsorption

temperatures were chosen at either room temperature (23 uC)

or body temperature (37 uC) at pH 7.4; for PMAA-grafted

surfaces, the pH of protein solutions were 4 or 9 at 23 uC.

Because it is very difficult to calculate the absolute surface area

of SiNWAs, the amount of protein adsorption is expressed as

mg/disc (for one disc, the ‘‘apparent’’ surface area is 0.5 cm2).

Results and discussion

Characterization of SiNWAs modified with polymers

The general process for the formation of poly(NIPAAm) or

poly(MAA) brushes on SiNWAs (SiNWAs-poly(NIPAAm) or

SiNWAs-poly(MAA)) is illustrated in Scheme 1. First, the

SiNWAs were prepared by a chemical etching method, as

previously reported.55 The length and diameter of the resulting

SiNWAs depended on the etching time; homogeneous SiNWAs

were prepared with an optimized etching time (Fig. S1, ESI{).

The initiator was immobilized, followed by SI-ATRP of

NIPAAm or tBMA using these surfaces as substrates. The

poly(tBMA) chains were further hydrolyzed in acidic solution

to obtain the corresponding poly(MAA) chains. For com-

parison, polymers grafted onto smooth silicon surfaces

(Si-poly(NIPAAm) and Si-poly(MAA)) were also prepared

following the same procedures. The resulting poly(NIPAAm)-

and poly(MAA)-grafted layers had a thickness of y39.2 nm and

y18.5 nm, respectively, as determined by ellipsometry. These

values are consistent with the values reported elsewhere.40,56

Changes in the chemical composition of SiNWAs after the

grafting of the polymers were determined from XPS data. The

survey spectra of the poly(NIPAAm)- and poly(MAA)-grafted

surfaces are shown in Fig. 1, and the corresponding chemical

composition of these surfaces is summarized in Table 1. The

experimental atomic ratios for the resulting surfaces were close to

the theoretical values, suggesting a successful modification process.

The nanostructures and surface morphology of unmodified and

modified SiNWAs were observed using SEM. The top view and

cross-sectional view are shown in Fig. 2, indicating a unique

nanoscale porous-type structure. All the nanowires were observed

to have similar diameters and lengths, and the majority were erect

and distributed evenly. The nanowires having diameters of about

100 nm and lengths of about 30 mm, combined together to form

Scheme 1 Process for grafting stimuli-responsive polymer

(poly(NIPAAm) or poly(MAA)) from SiNWAs.

Fig. 1 XPS survey spectra of (a) SiNWAs-poly(NIPAAm) and (b) SiNWAs-poly(MAA) surfaces.

Table 1 Experimental and theoretical elemental composition of poly-mer-grafted SiNWAs determined by XPS using 90u takeoff angles.

Surface C N O C/O

SiNWAs-poly(NIPAAm) 74.6 12.2 13.2 5.65theory 75.0 12.5 12.5 6.00SiNWAs-poly(MAA) 65.1 0.8 34.1 1.91theory 66.7 — 33.3 2.00

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nanoclusters. The unique nanoscale topography was preserved

after polymer modification.

Stimuli-responsivity of surface wettability

Stimuli-responsive polymers can display a reversible expanded/

collapsed conformational transition in response to environmen-

tal stimuli changes.57 When they are immobilized onto a solid

surface, the conformational change of chains will definitely

influence the surface properties and typically will lead to tunable

surface wettability.58,59 Based on many previous reports, the

introduction of nanoscale roughness onto a surface will

significantly affect surface wettability.60,61 Therefore, it is of

interest to investigate the influence of the nanostructure of

SiNWAs on the wettability.

Fig. 3a shows the water contact angle of Si-poly(NIPAAm) and

SiNWAs-poly(NIPAAm) surfaces as a function of temperature.

For both surfaces, an obvious jump in the water contact angle

occurred between 31–35 uC. The thermo-responsive wettability

change was significantly enhanced by the nanotopography of

SiNWAs. As indicated in Fig. 3b, as the temperature increased

from 23 uC to 37 uC, the difference in water CA value was y14u for

the Si-poly(NIPAAm) surface and y120u for the SiNWAs-

poly(NIPAAm) surface. This nano-enhanced thermo-responsive

wettability was consistent with another report.24

Because further protein adsorption was carried out in PBS

solution, it was important to investigate the surface wettability in

the liquid phase. In this work, two methods were used: the

captive bubble method to measure the water contact angle and

the sessile drop method to measure the oil (CH2Cl2) contact

angle in PBS. These tests were conducted under various tem-

peratures, and the results are summarized in Table 2. It is

suggested that the oil contact angle reflects the surface hydro-

philicity in the wetted state because a hydrophilic surface in a

water/air/solid system shows oleophobic behavior in an oil/

water/solid system.62 In PBS, the two surfaces exhibited different

wettability. For the SiNWAs-poly(NIPAAm) surface, the CA

value was almost unchanged as the temperature increased,

suggesting that the thermo-responsivity weakened or even

disappeared. The surface showed superhydrophilic and super-

oleophobic properties independent of temperature. We believe

this phenomenon results from the nanoscale porous structure of

SiNWAs, which harbors water molecules.24 From another point

of view, it is suggested that in aqueous solution the grafted

poly(NIPAAm) chains will swell and exhibit an extended

conformation. These swollen chains may ‘‘block up’’ the gaps

Fig. 2 SEM images of (a), (b) unmodified and (c), (d) polymer modified

SiNWAs. (a) and (c): top view; (b) and (d): cross-sectional view.

Fig. 3 (a) Water contact angle of Si-poly(NIPAAm) and SiNWAs-poly(NIPAAm) surfaces as a function of temperature. The typical contact

angle images for these surfaces at both 23 uC and 37 uC are shown in (b). Each value is the average of six parallel measurements. The standard error is

less than 2u.

Table 2 Contact angles for Si-poly(NIPAAm) and SiNWAs-poly(NIPAAm) surfaces under different conditions.

contactangle (u) a

Si-poly(NIPAAm)

SiNWAs-poly(NIPAAm)b

at 23 uC in air 58.1 0in PBS (bubble) 39.7 -in PBS (CH2Cl2) 129.2 145.6

at 37 uC in air 72.1 118.7in PBS (bubble) 43.7 -in PBS (CH2Cl2) 110.3 151.2

a Each value is the average of six parallel measurements. The standarderror is less than 2u. b - indicates that it was difficult to adhere thebubble to the surface.

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between the silicon nanowires, resulting in the transition from a

rough surface to a smooth one. This transition may also

contribute to the specific wettability of the SiNWAs-

poly(NIPAAm) surface in PBS.

We also investigated the influence of nanostructure on the

wettability of poly(MAA) grafted surfaces in response to pH.

Poly(MAA) is a typical weak polyacid, and the change in pH will

control the extent of ionization and further conformational

change of the polymer chains.47 As shown in Fig. 4a, there was

a concomitant decrease in the CA value of the smooth Si-

poly(MAA) surface as the pH increased from 4 to 6; a further

increase in pH led to an obvious drop in CA value from y45u to

y15u. This enhanced hydrophilicity of the poly(MAA) brush in

response to increasing pH has been previously reported and

can be explained by a conformational transition of the polymer

chains. 51 For poly(MAA), the addition of a base will

deprotonate the pendant acidic groups, resulting in swollen

polymer chains with many ionized COO2, which favors the

entrance of water molecules and makes the surface more

hydrophilic. However, in the case of the SiNWAs-poly(MAA)

surface, the pH-responsive wettability disappeared because the

surface remained superhydrophilic (with a CA value of y0u).The introduction of nanoscale roughness is believed to make the

surface superhydrophilic independent of pH; in the measured pH

range, the CA value of the flat Si-poly(MAA) surface was lower

than 65u, suggesting a hydrophilic surface.63 The CA values

measured in PBS also indicated the superhydrophilicity and

superoleophobicity of SiNWAs-poly(MAA) surface (Table 3).

Both the air bubble and oil drop easily rolled on the surface and

were difficult to adhere (Fig. S2, ESI{).

Stimuli-responsivity of protein adsorption

The results of the contact angle measurements show that the

combination of nanostructure and smart polymers will endow

the modified polymer with novel surface wettability in response

to environmental stimuli. It is well known that surface

wettability is one of the most important factors for protein

adsorption on biomaterial surfaces.64,65 Therefore, we further

measured protein adsorption on these modified surfaces under

different conditions to investigate the effects of surface

nanostructure and stimuli-responsive conformational change

on polymer chains. Fibrinogen (Fg) was used as a model protein

because it is abundant in plasma and plays a major role in blood

coagulation.66,67

Using poly(NIPAAm)-modified surfaces, adsorption from

1 mg mL21 Fg in PBS solution was measured at both room

temperature (23 uC) and body temperature (37 uC). As shown in

Fig. 5a, as the temperature increased, the adsorption on the

smooth silicon surface increased y10%, whereas that of the Si-

poly(NIPAAm) surface increased y110% because of the enhanced

hydrophobicity resulting from the thermo-responsive transition of

poly(NIPAAm) chains.40 From another point of view, the Si-

poly(NIPAAm) surface exhibits good protein-resistant properties.

The adsorption onto one disc (the surface area of one disc is

0.5 cm2) is y50 ng even at 37 uC, corresponding to a reduction

of y90% as compared with the unmodified Si surface. This

antifouling property of the poly(NIPAAm) layer is consistent with

previous reports.68,69 The introduction of nanoscale roughness

onto the smooth samples resulted in a significant increase in

surface area, which led to an obvious increase in the amount of

adsorption as compared with the unmodified Si surface. However,

it is interesting that there was no distinct difference in protein

adsorption between the Si-poly(NIPAAm) and SiNWAs-

poly(NIPAAm) surfaces. Compared with the SiNWAs surface,

the SiNWAs-poly(NIPAAm) reduced more than y99% of Fg

Fig. 4 (a) Water contact angle of Si-poly(MAA) and SiNWAs-poly(MAA) surfaces as a function of pH. The typical contact angle images for these

surfaces at both pH 4 and pH 9 are shown in (b). Each value is the average of six parallel measurements.

Table 3 Contact angles for Si-poly(MAA) and SiNWAs-poly(MAA)surfaces under different conditions.

contact angle (u) a Si-poly(MAA)b SiNWAs-poly(MAA)b

at pH 4 in air 48.9 0in PBS (bubble) 29.8 -in PBS (CH2Cl2) 136.4 –

at pH 9 in air 12.1 0in PBS (bubble) - -in PBS (CH2Cl2) – –

a Each value is the average of six parallel measurements. The standarderror is less than 2u. b - indicates that it was difficult to adhere thebubble to the surface. – means the surface is superoleophobic, with aCA value above 150u

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adsorption, regardless of temperature (Fig. 5b). It is suggested that

this high protein resistance resulted from the introduction of this

nanoscale structure. More water molecules were trapped in the

interstices of the nanowire arrays and formed a strong hydration

layer, which prevents intimate molecular contact between proteins

and the surface. This water-trapping effect was clear from the

contact angle (Table 2) and adhesive force measurements24 in

aqueous solution. Recently, Chen et al. found that SiNWAs-

poly(NIPAAm) surfaces showed largely reduced platelet adhesion

in vitro both below and above the LCST.24 This phenomenon can

be explained by the reduction of Fg adsorption on the surface

because it is well accepted that Fg plays a crucial role in platelet

adhesion and activation.66 To investigate this antifouling aspect

further, we measured the adsorption of two other typical proteins,

lysozyme and human serum albumin (HSA), with very different

charge and size70,71 (Fig. S3, ESI{). These results show that the

SiNWAs-poly(NIPAAm) surface exhibited effective nonspecific

protein resistance. Therefore, this novel surface is a promising

candidate for biomaterial and biomedical applications when

nonfouling properties are required.

The possible pH-responsive protein adsorption on poly(MAA)-

modified surfaces was also investigated. It was shown that Fg

adsorption on smooth silicon surfaces at pH 9 was slightly lower

than at pH 4, whereas after modification with poly(MAA), the

effect of pH was greater. The adsorption at pH 4 was y38 times

higher than that at pH 9, but the amount of adsorbed Fg is still

lower than 4 mg/disc (Fig. 6a). The introduction of nanoscale

roughness onto the smooth Si substrate led to an obvious increase

in the amount of adsorption as a result of the enlarged surface area,

but the pH-responsiveness was not enhanced. Interestingly, the

combination of pH-sensitivity of poly(MAA) chains and the nano-

effects of 3D nanostructured SiNWAs gave the SiNWAs-

poly(MAA) material an extremely high capacity for binding Fg

at pH 4, with adsorption greater than 70 mg per disc, i.e., a more

than 70-fold increase compared with smooth silicon surface and a

y20-fold increase compared with smooth Si-poly(MAA) (Fig. 6b).

As shown in Fig. 6c, the differences in adsorbed protein and the

ratio of adsorbed quality between pH 4 and pH 9 on SiNWAs-

poly(MAA) surface were greatest. This significant pH-sensitive

protein adsorption may be attributed to the synergistic effect

between the conformational transition of poly(MAA) chains and

nano-enhanced effects of SiNWAs.19,72 At pH 4, the poly(MAA)

chains are collapsed and more hydrophobic, and hydrogen bonds

might be formed between the -COOH groups on poly(MAA)

chains and -CONH groups on proteins, resulting in an increase in

protein adsorption. In addition, the collapsed poly(MAA) chains

may favor access of Fg to the interstices of the nanowire arrays. At

pH 9, the -COOH groups are ionized to -COO2 groups, resulting

in extended hydrophilic chains with negative charges. Because Fg

also carries negative charges, a reduction in hydrophobic interac-

tion and enhanced electrostatic repulsion lead to reduced protein

adsorption at pH 9. On the other hand, it is assumed that

interactions between Fg and poly(MAA) are significantly

enhanced by the nanostructure of SiNWAs. Water molecules are

favored to be trapped in the interstices of the nanowire arrays,

giving a high degree of hydration and reducing protein adsorption,

especially under basic conditions at pH 9. This result is consistent

with our previous report,26 suggesting that the introduction of

nanoscale structure onto poly(MAA)-modified surfaces leads to

significantly enhanced pH-responsivity of protein adsorption,

although the pH-responsive wettability is weakened.

The significant adsorption difference motivated us to question

whether the Fg adsorbed at pH 4 can be desorbed by increasing

the pH. To that end, SiNWAs-poly(MAA) samples were allowed

to adsorb Fg at pH 4. They were then incubated in PBS at pH 9

for 3 h, and the quantity of the remaining adsorbed Fg was

measured. For one disc, more than 70 mg (corresponding to

y96%) adsorbed Fg was released from substrate into solution

by simply increasing the pH (Fig. 6d). Such pH-controlled

binding of proteins is of great importance for many biomedical

applications, including protein delivery and bioseparations.73,74

The potentially reversible process of adsorption and release by

cycled pH change will be tested in a future study.

Conclusion

In summary, two stimuli-responsive polymers, poly(NIPAAm)

and poly(MAA) were grafted from initiator-immobilized SiNWAs

with nanoscale topography. The two resulting polymer-coated

surfaces exhibited different wettability and protein adsorption

in response to environmental stimuli. For the SiNWAs-

poly(NIPAAm) surface, changes in temperature from 23 uC to

37 uC lead to a significant change in wettability in the dry state

Fig. 5 Adsorption of 1 mg mL21 fibrinogen from PBS solution over a 3 h period on (a) pristine and poly(NIPAAm)-modified silicon surfaces and (b)

pristine and poly(NIPAAm)-modified SiNWAs surfaces at 23 uC and 37 uC. For SiNWAs samples, the ‘‘apparent’’ surface area of one disc is 0.5 cm2.

Data consist of the mean ¡ standard error (n=3).

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(from superhydrophilic (water CA y0u) to strongly hydrophobic

(water CA y120u)), but no obvious difference in protein

adsorption. This surface exhibited good non-specific protein

resistance regardless of temperature, probably because the

nanoscale topography bound many water molecules to form a

strong hydration layer. In contrast, for SiNWAs-poly(MAA)

surface, the introduction of nanoscale roughness resulted in a

superhydrophilic surface in the treatment pH range (4 to 9),

suggesting the disappearance of pH-responsive wettability.

However, the pH-responsivity of protein adsorption was enlarged.

These results indicated that the introduction of the same nanoscale

structure onto surfaces modified with different stimuli-responsive

polymers leads to different effects on surface properties. Moreover,

for surfaces with nanotopography, surface wettability and

bioadhesion may not show consistent trends. This surface showed

an extremely high capacity for binding protein at pH 4, and most

of the adsorbed protein can be released by simply increasing pH.

Based on our results and taking into account the good

biocompatibility of SiNWAs, we believe that these surfaces will

find applications in biomedical devices, protein delivery and

biosensors. In addition, the work presented here also offers a new

strategy for designing and fabricating other nanomaterials with

special properties.

Acknowledgements

This work was supported by the National Natural Science

Foundation (21074083, 20974086, and 20920102035). We are

grateful to Professor Hongwei Ma’s group for assistance with

SEM measurements.

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