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Colloids and Surfaces A: Physicochem. Eng. Aspects 360 (2010) 20–25

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

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dsorption of poly(l-lysine) on silica probed by optical reflectometry

in Jiang1, Ionel Popa2, Plinio Maroni ∗, Michal Borkovecepartment of Inorganic, Analytic, and Applied Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1205 Geneva, Switzerland

r t i c l e i n f o

rticle history:eceived 26 October 2009eceived in revised form 27 January 2010

a b s t r a c t

Optical reflectometry was used to study adsorption behavior of poly(l-lysine) (PLL) on the water–silicainterface for different pH and concentrations of the monovalent salt NaBr. At low salt level and low pH,the adsorbed amount increases with increasing salt concentration and pH. This adsorption behavior can

ccepted 29 January 2010vailable online 6 February 2010

eywords:dsorption

be qualitatively explained by electrostatic interactions between the negatively charged substrate and thepositively charged polyelectrolyte. At higher salt levels and pH, the adsorbed amount passes through amaximum. This maximum is probably related to the competition between adsorbing monovalent cationsand the polyelectrolyte. The initial adsorption kinetics is mainly controlled by the mass transport to thesurface.

oly(l-lysine)

eflectometryater–silica interface

. Introduction

Due to its biocompatibility, poly(l-lysine) (PLL) has foundidespread use in various surface modification protocols [1–8].n important application of PLL of this kind involves the fabri-ation of biocompatible polyelectrolyte multilayers, especially inonjunction with a biocompatible anionic polyelectrolyte, such as,yaluronic acid [1–4]. Such coatings have interesting pH responsiveehavior, they can be effective to control cell adhesion, and theyay protect surgical implants from bacteria. Another promising

pplication is its use as a backbone of a comb-like polyelectrolyteith poly(oxyethylene) (PEO) side chains [5,6]. The PLL backbone

erves to anchor the polyelectrolyte on a negatively charged sur-ace, while the PEO side chain forms a dense brush covering theurface. Surfaces modified in this fashion show remarkable proteinepellency, which can be essential for cell culture growth or sensorevelopment.

The solution properties of PLL have been intensively studied inhe past. This weak polybase is fully charged in acidic and neu-ral region, and deprotonates in the pH region 9–11 as has been

stablished by potentiometry titration, spectroscopic techniques,nd modeling tools [9,10]. Simultaneously, PLL undergoes a con-ormational transition from a random coil state to a �-helix asvidenced by circular dichroism measurements [11,12]. Dynamic

∗ Corresponding author. Tel.: +41 223796421; fax: +41 223796069.E-mail address: Plinio.maroni@unige.ch (P. Maroni).

1 Present address: Institute of Physical und Theoretical Chemistry, University ofonn, Bonn, Germany.

2 Present address: Department of Biological Sciences, Columbia University, Nework City, NY 10027, USA.

927-7757/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2010.01.070

© 2010 Elsevier B.V. All rights reserved.

light scattering and viscosity measurements reveal that PLL adoptsan extended coil conformation at low pH. At higher pH, the �-helixconformation becomes more prominent and at higher tempera-tures, the chain adopts a �-sheet structure [13–15].

Even though the mentioned applications of PLL rely on thestrong interaction of this polyelectrolyte with water–solid inter-faces, relatively little is known about its adsorption behavior[16–20]. Adsorption of PLL was studied to negatively chargedlatex particles in some detail [16–18]. These studies concludedthat at neutral conditions the adsorbed amount is in the range of0.3–0.6 mg/m2, and that it increases with increasing ionic strength.Surface plasmon resonance and fluorescence techniques wereused to investigate adsorption of PLL on differently functional-ized self-assembled monolyers [8]. The results suggest relativelythin adsorption layers and widely differing adsorption densitiesdepending on the functionalization of the substrate. The limitedresults available for silica suggest that the adsorbed amount of PLLon this surface is lower than that on latex particles [17].

This article provides a complete study of the adsorption of PLLon silica surface at different pH and ionic strengths by in situ reflec-tometry. This technique is ideally suited to obtain detailed resultson the adsorption kinetics and adsorbed amount at saturation. Thepresent results demonstrate that its adsorption behavior is stronglyinfluenced by the helix–coil conformational transition and to alesser extent by the ionic strength.

2. Experimental

2.1. Materials

PLL hydrobromide with a molecular mass of 300 kg/mol waspurchased from Fluka. All solutions were prepared with Milli-Q

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The adsorbed mass increases first in a linear fashion with time,and the slope is related to the initial adsorption rate. This slope canbe well estimated by plotting the time derivative of the adsorptiontransient (see Fig. 2). A constant initial slope is reflected by a plateau

M. Jiang et al. / Colloids and Surfaces A:

ater, their pH was adjusted with either HCl or NaOH, and theironic strength with NaBr. Silica surfaces were obtained by heatingilicon wafers (p-type, boron doped, Silchem, Freiberg, Germany)n a furnace at 1000 ◦C for 6 min. The thermally grown silica layern silicon wafer was uniform with a thickness of about 20 nm. Therecise thickness of the silica layer was determined in air by null-llipsometry (Multiskop, Optrel) [21]. The data were analyzed withslab model whereby the refractive index of silicon and silica usedas fixed at 3.85–0.02 i and 1.457, respectively. Subsequently, theafers were cut into pieces of about 2 cm2 and cleaned in a mixture

f 24% NH3, 30% H2O2, and water in a volume ratio of 1:1:5 at about0 ◦C for 10 min. The pieces were extensively rinsed and stored inilli-Q water prior to measurements.

.2. Reflectometry

The adsorption behavior of PLL on cleaned silica surface wasnvestigated in situ by reflectometry in a stagnation point flow cell22–24]. The home-made reflectometer used is equipped with a He-e laser emitting at a wavelength of 632.8 nm. The intensity of the

aser beam is stabilized with the two-mode frequency stabilizationechnique [25]. The intensity was stable within 0.5% after 1 h ofperation. Before entering the cell, the light beam passes through ainear polarizer. The cell consists of a capped equilateral dispersingrism made out of quartz (CVI optics, EDP-25.0-UV) and it has aorehole with a radius of r = 0.5 mm used to inject the solution.wo spacers ensure a horizontal gap between the surface and therism of h = 0.85 mm. The laser beam passes through the prism andeflects from the surface at the stagnation point with an incidencengle of 71.3◦. The reflected light beam is separated into its parallelp) and perpendicular (s) components through a polarizing beamplitter. The intensities of the two light beams originating from theolarizing beam splitter are monitored with two photodiodes andre proportional to the respective reflectances R(p) and R(s). Furtheretails on the setup used here are given elsewhere [24].

The reflectometry signal R is related to the reflectances by= CR(p)/R(s) where C is an unknown instrumental constant thatccounts for losses at the surfaces of the optical components andor differences between the two photodiodes. This constant can beliminated by normalizing the signal to its initial value

(t) = R(t) − R(0)R(0)

(1)

here t = 0 defines the beginning of the experiment. The normal-zed reflectometry signal is proportional to the adsorbed mass �n the surface [22]

(t) = S(t)A

(2)

here A is the sensitivity factor. This factor can be calculated fromhomogeneous slab model with Abeles’ matrix formalism [21,22].he needed refractive index of the adsorbed layer n with a thicknessfollows from the simple mixing law [22]

= nw + �

L· dn

dc(3)

here nw is the refractive index of water and dn/dc is the refrac-ive index increment of a PLL solution. For a typical silica layerith a thickness of 20 nm, the sensitivity factor A is 0.15 m2/mg.

he refractive index increment of a PLL solution is 0.18 mL/g [4],

nd very similar values were reported by others [26,27]. This valuef the refractive index increment was verified to be independentf pH and ionic strength in our laboratory. The reproducibility inhe adsorbed mass for two consecutive measurements performednder identical conditions is better than 15%.

ochem. Eng. Aspects 360 (2010) 20–25 21

3. Results and discussion

Adsorption of PLL at water–silica interface was studied by insitu optical reflectometry. A typical experiment proceeds as fol-lows. A surface mounted in the impinging jet cell was flushedwith a background solution with given pH and NaBr concentra-tion. The reflectometry signal recorded during this initial periodof about 5 min represents the baseline. Subsequently, the inputsolution is switched to a PLL solution with the same pH and ionicstrength as the background solution. This point defines the startof the experiment (t = 0). Initially, the adsorbed mass increases lin-early as a function of time. This part reflects the initial kinetics of theadsorption process. After a transient period, an adsorption plateauis observed. This plateau corresponds to the saturation under thespecific solution conditions. Typical results of such experiments fordifferent PLL concentrations in a solution of pH 5.9 and an ionicstrength of 10 mM adjusted with NaBr are shown in Fig. 1. Sincethe flux to the surface is proportional to the PLL concentration, theinitial slope of the adsorption process increases accordingly. Theadsorption plateau is the same for PLL concentrations ≥1 mg/L, butat lower polymer concentrations, the saturation plateaus are notvisible within the experimental time window as shown in Fig. 1.However, the corresponding plateau values are reached after longeradsorption times. Similar experiments were performed for PLLsolutions with pH between 4 and 11 and ionic strengths between0.1 mM and 1 M.

Desorption experiments were equally carried out for the con-ditions mentioned. Once the adsorption plateau has been reached,the cell was flushed with the same polyelectrolyte-free backgroundsolution, which was initially used to condition the surface prior toadsorption. No desorption of PLL was observed for the conditionsinvestigated.

The results will be discussed in two parts. The first part accountsfor the initial adsorption kinetics, while the second part describesthe adsorbed amount at the saturation plateau.

3.1. Initial adsorption rate

Fig. 1. Adsorbed mass of poly(l-lysine) (PLL) on water–silica interface at pH 5.9 andat an ionic strength of 10 mM in NaBr measured by time-resolved in situ reflec-tometry at different polymer concentrations. The inset shows the repeating unit ofPLL.

22 M. Jiang et al. / Colloids and Surfaces A: Physic

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ig. 2. Adsorption rate of PLL on silica at pH 5.9 and at an ionic strength of 10 mMn NaBr at different polymer concentrations. The plateaus reflect the initial constantdsorption rate d� /dt|t=0.

n the derivative plot. At lower concentrations, such plateaus areell developed, and the corresponding adsorption rates can be esti-ated to good accuracy. At higher concentrations, however, the

stimation of the plateau values becomes more problematic due tots short duration and the fact of being masked by the finite deadime of the cell, which is estimated to be about 10 s. Nevertheless,he initial adsorption rates can be estimated to good confidence inconcentration range of about 0.1–10 mg/L of PLL. The results are

hown in Fig. 3. The data confirm that the adsorption process is ini-ially of first order in the PLL concentration c, thus confirming the

ate law

d�

dt= kadsc (4)

ig. 3. Initial adsorption rate d� /dt|t=0 as a function of the PLL concentration at pH.9 and at an ionic strength of 10 mM in NaBr. The solid line reflects the first orderinetics with respect to the solution concentration and an adsorption rate coefficientf kads = 1.3 �m/s.

ochem. Eng. Aspects 360 (2010) 20–25

where kads is the adsorption rate coefficient. The solid line in Fig. 3corresponds to this proportionality relation given by Eq. (4) andleads to an adsorption rate coefficient of kads = 1.3 �m/s.

The dependence of the adsorption rate coefficient kads on thesalt concentration is shown in Fig. 4 for different pH values. ThepH dependence is weak and within experimental error. However,the coefficient increases with the ionic strength. This behavior canbe explained by considering the change in conformation of thePLL chains. With increasing ionic strength, the repulsion betweencharged residues is progressively screened, and the chain becomesmore compact. More compact chains diffuse faster through thestagnation layer near the surface, leading to a higher flux to thesurface and a larger adsorption rate.

This effect can be quantified by comparing the adsorption rateto the theoretical value for the impinging jet geometry with perfectsink conditions. The simplest approach is to consider the stick-ing coefficient ˇ defining the ratio between the experimentallyobserved absorption rate coefficient kads and its theoretical value,namely

ˇ = kads

k(0)ads

(5)

where k(0)ads represents the theoretical adsorption rate coefficient for

perfect sink conditions. For the impinging jet geometry used here,this rate coefficient is given by the expression due to Dabros andvan de Ven [28]

k(0)ads

= 0.776r

(�D2 ¯ Re

)1/3(6)

where r is the radius of the bore hole in the prism,� � 8.94 × 10−7 m2/s is the kinematic viscosity of water, D is thediffusion coefficient of a PLL molecule, Re = ur/� is the Reynoldsnumber, and ¯ is the dimensionless flow intensity parameter. Themean flow velocity of the fluid in the tube u is determined fromthe volumetric flow rate dV /dt = �r2u. The dimensionless flowintensity parameter ¯ has been calculated for the impinging jetgeometry. For a volumetric flow rate of 0.92 mL/min one findsRe � 9.8, and with h/r � 1.7 the flow intensity parameter becomes

¯ � 5.4 [28]. The diffusion coefficient D of the PLL molecules is nor-mally expressed as the hydrodynamics radius Rh. These quantitiesare related with the Stokes–Einstein relation

D = kBT

6��Rh(7)

Fig. 4. Adsorption rate coefficient of PLL on silica as a function of the ionic strengthin NaBr and for different pH values.

M. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 360 (2010) 20–25 23

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3

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ig. 5. Sticking coefficient of PLL on silica as a function of the ionic strength in NaBrnd for different pH values. A sticking coefficient ˇ = 1 corresponds to adsorptioninetics limited by mass transfer.

here kB is the Boltzmann constant, T is the absolute temperature,� 8.91 × 10−4 Pa s is the dynamic viscosity of water. The value ofh � 18 nm was found for PLL of similar molecular mass at a NaBralt concentration of cS = 0.2 M by dynamic light scattering [14],nd the relation Rh ∝ c−0.17

S was used to extrapolate this quantityo other salt concentrations [29].

The sticking coefficients obtained in this fashion are showns a function of ionic strength in Fig. 5 for different pH values.he resulting values of ˇ = 0.48 ± 0.07 are relatively independentf the ionic strength above 1 mM. At the lowest ionic strengthsnvestigated, however, the sticking coefficient becomes substan-ially larger. This increase can be explained by the enhancementf the electrostatic attraction between the positively charged PLLnd negatively charged substrate. Similar increase of the stickingoefficient at low ionic strengths has been reported for the deposi-ion of colloidal particles to oppositely charged substrates and foreteroaggregation [30,31]. The observed mass transport limitationf the adsorption process is consistent with other studies of poly-lectrolyte adsorption on oppositely charged substrates [32–35].

.2. Adsorbed mass at saturation

For long adsorption times, the adsorbed mass saturates at alateau. The influence of the ionic strength and solution pH on theaturation plateau values will be discussed in the following.

Fig. 6a shows the adsorbed mass at saturation as a function ofalt concentration for different solution pH. At low pH, the adsorbedmount increases with increasing ionic strength. The presentlyeported adsorbed amounts are somewhat lower than those mea-ured for colloidal silica at pH 4 [17]. However, both data setsre well comparable, and show very similar trend with the ionictrength. The amount of PLL adsorbed to negatively charged latexarticles is about 3–5 times larger [17,18]. The reported increaseith the ionic strength further resembles the cases of adsorption

f poly(diallyldimethyl ammonium chloride) (PDADMAC) [35,36],

oly(amido amine) (PAMAM) dendrimers [34], and other cationicolyelectrolytes [32,37–40]. At higher pH, however, the adsorbedmount still increases at low ionic strength, but at higher ionictrength passes through a maximum. This maximum is locatedetween 0.1 and 1.0 M, and shifts slightly towards lower ionic

Fig. 6. Adsorbed amount for PLL on silica at the saturation plateau in NaBr. Datapresented as a function of (a) the ionic strength and (b) pH. The solid lines serve toguide the eye only.

strengths with increasing pH. Fig. 6b illustrates that the maximumis equally apparent as a function of pH at an ionic strength of 1 M.The latter figure equally shows that the adsorbed amount increasesabruptly between pH 9 and 10. The adsorbed amount ranges from0.05 to 0.8 mg/m2 which corresponds respectively to surface den-sities of 1 × 1014 and 16 × 1014 molecules/m2.

The observed behavior can be explained tentatively as follows.While PLL is a weak polybase, it is fully charged at pH < 9 [9,10].At low salt levels, the highly charged polyelectrolyte chains repelstrongly due to electrostatic interactions, and lead to a looselypacked adsorbed monolayer. With increasing ionic strength, therepulsive forces are getting progressively screened, leading toweaker repulsion and higher adsorption densities. This behaviorhas been rationalized with random sequential adsorption (RSA)model [34,35].

However, this picture is only applicable to a weakly charged sub-strate, such as silica at low pH. With increasing pH, silica becomes

progressively charged due to the dissociation of silanol groups.Thereby, an increasing amount of cations are accumulated in itsdiffuse layer to neutralize the negative silica charge. These cationscontribute additionally to the screening of the repulsion betweenthe charged polyelectrolytes, leading to increase of the adsorption

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ith increasing pH. Based on this mechanism, a quantitative expla-ation of the adsorbed amount of PAMAM dendrimers on silicaould be given [34], and the amount of adsorbed PDADMAC coulde explained semi-quantitatively [35].

The sharp increase in the adsorbed amount between pH 9 and0 (see Fig. 6b) can be explained by the deprotonation of PLL, whichappens precisely in this pH window [9,10]. The deprotonation isccompanied by a conformational change from random coil to an-helix and a compaction of the chain [10,11,13,14]. In this region,

he hydrodynamic radius of PLL molecules decreases substantially14]. This compaction of the chain leads to a larger number densityt saturation. This transition appears similar to the one observedor grafted PLL [41]. Interestingly, the adsorption of the weakationic polyelectrolyte poly(dimethylamino ethyl methacrylate)DMAEMA) silica showed a similar decrease with increasing pH andonic strength [42]. Other weakly charged polyelectrolytes behaveimilarly [37,39,43,44]. At high pH and low ionic strength, thedsorbed amount depends only weakly on the ionic strength. Atigh ionic strength, the adsorbed amount decreases significantly.his decrease can be explained with the competition betweenonovalent cations originating from the background solution and

he weakly charged PLL [43,45,46]. Alternatively, this phenomenonight be equally related to the time-dependence of the spreading

f the adsorbed polymers [47,48].The film structure cannot be inferred by reflectometry measure-

ents alone. However, at pH smaller than 9, PLL is highly chargednd one would expect this polyelectrolyte to behave similarly toDADMAC [35]. Based on this, at low pH and low ionic strength, PLLhould adsorb on silica in a flat conformation, giving rise to homo-eneous films. For larger values of pH and high ionic strength, PLLeprotonates and acquires an �-helix structure. Thus, one wouldxpect PLL to have a more “globular” shape on the surface.

. Conclusion

Adsorption of PLL to water–silica interfaces was studied byptical reflectometry at different ionic strengths and pH. At lowonic strength and low pH, the adsorbed amount increases withncreasing salt concentration and pH. This adsorption behavioran be qualitatively explained by electrostatic interactions andeminds of similar trends observed during adsorption of otherighly charged polyelectrolytes on silica. At high ionic strength andigh pH, however, the adsorbed amount passes through a maxi-um. This maximum can be eventually explained by competition

etween the adsorption of monovalent cations originating from theackground solution and the polyelectrolyte. Similar adsorptionaxima were observed in other weakly charged polyelectrolytes.

he initial adsorption rate is of first order with respect to the poly-lectrolyte concentration and the adsorption kinetics is close to theransport limited behavior.

cknowledgements

This work was supported by Swiss National Science Foundationnd the University of Geneva.

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