Article pubs.acs.org/JPCC
Shape Recognition of Nanoparticle-Imprinting Materials Enhanced by Depletants Yen-Fu Chen,† Hsin-Ju Tsai,† Yu-Jane Sheng,*,† and Heng-Kwong Tsao*,‡ †
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China Department of Chemical and Materials Engineering, Department of Physics, National Central University, Jhongli, Taiwan 320, Republic of China
‡
ABSTRACT: The recognition of nanoparticles by imprinted materials via entropic depletion attraction is investigated by dissipative particle dynamics simulations. It is found that the depletion attraction exits between nanoparticles and imprinted materials with complementary shapes based on the interaction energy U(H) and association energy Ea. The strength of the attractive depletion grows with increasing size of a perfectly matched target/cavity (TC) pair owing to the increment in their overlapping excluded volumes. The uptake of targets can be significantly enhanced by increasing the concentration of depletant ϕD. The selective recognition between perfectly matched and mismatched TC pairs is also studied, and a very high selectivity can be achieved at an optimal ϕD. The kinetics of the recognition process reveals that small nanoparticles migrate fast and access the cavity easily but move out of the cavities eventually due to their weak association energy. Finally, the synergetic effect of entropic depletion and enthalpic affinity is proved to enhance the association fraction substantially for small perfectly matched TC pairs with weak affinity. Our simulation results demonstrate the importance of the depletion effect on the nanoparticle-imprinting technology. shell. Thin nanoparticle-imprinted polymer films may act as the recognition element for efficient and fast sensing devices.13,14 Although some methods such as dynamic light scattering and electrochemical detection have been employed for nanoparticle characterization, these techniques are generally complex and expensive. Nanoparticle’s imprinting with specific cavities responding to the structural properties of nanoparticles shows promise for screening and separation, in addition to being suitable for field analysis. In general, it is believed that the uptake of macromolecules or nanoparticles by the surfaces of MIPs or nanoparticleimprinted polymers (NIPs) is intrinsically driven by the enthalpic affinity of the binding sites, in addition to shape matching. Those enthalpic interactions include hydrogen bonding, dipolar, and electrostatic attractions.15−17 The selective attraction between targets and MIPs or NIPs is often described by the lock-and-key model to which Fischer’s lock-and-key principle applies.18,19 Recently, lock−key assemblies involving a spherical key fitting inside a lock cavity can be achieved in nonadsorbing polymer solution without any molecular bonding. That is, they are entropically driven and do not depend on their chemical composition and surface functionalities. Such a polymer-mediated attraction is resulted from the depletion force, suggested first by Asakura and
I. INTRODUCTION Molecular imprinting involves the creation of template-shaped cavities on the surface of polymeric matrices and has been used in molecular recognition for applications such as biomedical sensor,1−5 drug development and screening,6 and separation of products in chemical reactions.7 This technique is essentially based on enzyme−substrate recognition where the enzyme binds selectively to the substrate that has a corresponding geometric structure to the active binding sites. After the processes of imprinting and removal of templates, a molecularly imprinted polymer (MIP) possesses the memory of the template molecules and forms specific cavities which are complementary to the size and shape of templates. Thus, there is a binding affinity developed between MIPs and template particles. Over the years, the performance of the target recognition of MIPs has been systematically improved by studying the effects of reactant (functional monomer, crosslinker, initiator, porogenic solvent, and extraction solvent), polymerization conditions, and outer stimuli (photoirradiation, pH change, electric field, and magnetic field).8 Recently, by extending the well-known concept of molecular imprinting, nanoparticle’s imprinting has also been developed owing to the increasing interest in nanoparticles.9−11 Nanoparticles made of metals, oxides, and organics exhibit sizedependent physical and chemical properties and often attain toxicity.12 Nanoparticle’s imprinting can be used for selective recognition of nanoparticles based on their size, shape, and © 2016 American Chemical Society
Received: July 19, 2016 Published: August 17, 2016 19871
DOI: 10.1021/acs.jpcc.6b07260 J. Phys. Chem. C 2016, 120, 19871−19877
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Figure 1. Schematic diagram of a system with nanoparticles, nanoparticle-imprinted materials, polymeric depletants, and solvents.
Ossawa.20,21 The emergence of depletion forces between two colloidal particles with shape complementarity is attributed to the overlap of their excluded volumes, which increases the system free volume accessible to solvents and polymeric depletants. Upon the formation of lock−key pairs, the increase of the entropy for solvents and depletants leads to the decrease of the system free energy. Consequently, such an entropic attraction induced by osmolytes depends only on size, shape, or surface roughness of colloids and has been applied to the shapeor size-dependent separation of nanoparticles.22−29 In general, the depletion force grows as the depletant concentration is increased.30 In experiments of recognition of molecules or nanoparticles, the effective attraction between targets and imprinted materials may be enhanced by depletion effects because all solutes other than targets can be essentially viewed as depletants. As the concentration of those other solutes, possessing weak affinity and shape complementarity, is increased, the uptake amount of targets may be elevated. On the other hand, even if the enthalpic affinity between targets and imprinted materials is weak, it is anticipated that the adsorption of targets can be raised by the addition of polymeric depletants as the shape complementarity between targets and imprinted materials is satisfied. In this work, dissipative particle dynamics (DPD) simulations are employed to investigate the recognition of nanoparticles by imprinted materials. It is generally regarded as an association process between the target (T) and the cavity (C) on NIP to form the complex (TC), T + C ⇌ TC, driven by strong enthalpic affinity. The role of the depletion force in nanoparticle’s imprinting is examined. The effect of depletant addition on the uptake of targets is studied as well. The selectivity based on shape complementarity only is demonstrated by considering the screening of a mixture of nanoparticles with different radii.
II. MODEL AND SIMULATION METHOD A coarse-grained particle-based DPD simulation has been used in this study.31−33 A cluster of atoms or molecules is represented by a DPD bead with mass m and diameter rc. All DPD beads obey Newton’s equation of motion. The force acting on a DPD bead includes conservative (FCij ), dissipative, and random forces. All of them are soft, pairwise-additive, and of the same interaction range rc. The thermodynamic behavior of the system is related to the conservative force between beads I and j, FCij = aij(rc − rij)r̂ij, where aij is the maximum repulsion and rc the cutoff radius. rij depicts the distance between the two beads, and r̂ij the unit vector in the direction of the separation. There are four kinds of DPD beads: solvent (S), nonadsorbing depletant (D), target (T), and cavity on imprinted material (C). The interaction parameters between the same species are always set as aii = 25. In order to illustrate the contribution of depletion, aij (i ≠ j) is generally chosen as 25 to yield an athermal system. Thus, aggregation among targets or depletants is prevented in a good solvent condition. As the enthalpic affinity comes into play, the interaction parameter between targets and cavities is varied as aTC = 18−24 to demonstrate the joint effort of the enthalpic affinity and entropic depletion effect. All the units are scaled by the bead mass m, cutoff distance rc, and thermal energy kBT. Targets (nanoparticles) and imprinted materials with complementary shapes are constructed by a cluster of DPD beads. Both are arranged in a body-centered cubic structure. As shown in Figure 1, the radius of the target is RT = 1.5, 2.0, or 2.8. A hemispherical cavity with the radius RC = 1.5 or 2.8 is created on the surface of the imprinted material by removing the DPD beads within the cavity. Depletant is also made by connecting a few DPD beads. The osmolyte depletant contains 3 beads (LD = 3) while the polymeric depletant has 12 beads (LD = 12). Beads forming targets and depletants are bound together by harmonic spring forces with the spring constant 100 and the equilibrium length 0.7. 19872
DOI: 10.1021/acs.jpcc.6b07260 J. Phys. Chem. C 2016, 120, 19871−19877
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acquired from the calculation of the potential of mean force.35 It is the average work required to bring a pair of target and cavity from infinity to the distance H. First, the depletion force is evaluated over all the configurations of all DPD beads of solvents and depletants in the system by keeping all the DPD beads of the target and the imprinted material at fixed positions.36 Then, U(H) can be evaluated by the integration of the mean force with respect to the their separation. The association energy (Ea) corresponds to the minimum value of U(H).
In order to reinforce the rigidity of the target, additional spring forces are imposed among nonadjacent beads.30,34 Typically, 98 pairs of targets and cavities on the imprinted material exist in our simulation system. The dynamics of all DPD beads with the total number Ntotal = 526 848 is performed in a cubic box (563) under periodic boundary conditions at the system density ρ = 3. Note that the beads forming the imprinted material are with fixed positions. The DPD time step is set as Δt = 0.04, and the total DPD steps are at least 3 × 106. The volume fraction of solvent varies from 0.40 to 0.8, and that of depletant changes from 0 to 0.40. The volume fraction of target is about 0.04. The definition of the volume fraction of a particular species is simply defined as the ratio of the total bead number of that species to Ntotal. The adsorbed complex TC is defined as the pair of target and cavity with their separation less than 0.2. As shown in Figure 2, the separation H is the distance between the lowest point of the target and the innermost surface of the cavity. The profile of the depletion potential U(H) between a pair of target and cavity can be
III. RESULTS AND DISCUSSION In addition to enthalpic affinity, the attraction between two particles may be caused by entropic depletion interaction. Consequently, one may wonder whether the association between targets and cavities of the imprinted material can be effectively driven by the depletion effect. In this work, first, the characteristics of entropic depletion between the target and the imprinted substrate are compared to those of enthalpic affinity. The effects of size, shape complementarity, and concentration of polymeric depletants on depletion are also investigated. Second, the equilibrium states of a nanoparticle dispersion exposed to an imprinted material are achieved, and the association fraction (θa) is therefore acquired quantitatively, where θa is related to the association energy. Third, to understand the selective recognition, the competitive adsorption of targets with different sizes on the same imprinted material is examined. Lastly, the synergetic effect of entropic depletion and enthalpic affinity is explored by considering a system consisting of pairs of targets and cavities with weak enthalpic affinity. The enhanced association is acquired by the addition of polymeric depletants. A. Characteristics of Attractive Depletion between Targets and Imprinted Substrates. The specific association site on the imprinted surface is generally corresponding to functional groups with hydrogen bonds or electrostatic attractions. While it is regarded as the main cause for the recognition of nanoparticles, the shape complementarity associated with the depletion effect is less discussed for the formation of the adsorbed complex. To compare the characteristics of entropic depletion with those of enthalpic affinity, the potential of mean force U(H) is evaluated for two different adsorption sites of the same radii, an enthalpically attractive circular patch (aTC = 20) and an enthalpically neutral cavity (aTC = 25). An enthalpically neutral circular patch with aTC = 25 is also considered for contrast. As demonstrated in Figure 2a, for all the U(H) profiles, there exist a free-energy barrier and a potential well, as the nanoparticle with RT = 2.8 approaches the site. The barrier originates from local solvent particle packing correlations, and the potential well is attributed to affinity or depletion attraction. For circular patches, the potential well becomes deeper as aTC is lowered from 25 to 20. The magnitude of the primary minimum of U is referred to as the association energy (Ea ≅ −Umin, scaled by kBT). Ea ≅ 7 corresponding to aTC = 20 is in the range of typical hydrogen bonds. For the cavity with RC = 2.8 complementary to the nanoparticle, the association energy is Ea ≅ 17, which is significantly greater than that of the attractive circular patch. This result suggests that entropic depletion also plays an important role in the adsorption of targets onto cavities of imprinted materials. The influences of shape complementarity on the depletion interaction are studied by considering two perfectly matched
Figure 2. (a) Potential of mean force (U) against the distance (H) with cavity (Rc = 2.8) and with circular patch (no cavity) for RT = 2.8 and ϕD = 0. Note that a = 25 means no special interaction between the nanoparticle and the substrate while a = 20 means that there is an enthalpic affinity between the nanoparticle and the circular patch on the substrate. (b) Potential of mean force (U) against the separation distance (H) for a small perfectly matched TC pair (RC = RT = 1.5), a large perfectly matched TC pair (RC = RT = 2.8), and mismatched TC pair (RC = 2.8, RT = 2.0) at ϕD = 0.1. 19873
DOI: 10.1021/acs.jpcc.6b07260 J. Phys. Chem. C 2016, 120, 19871−19877
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The Journal of Physical Chemistry C TC pairs with different sizes and a mismatched TC pair. The interactions between these targets and cavities are enthalpically neutral. Figure 2b shows the depletion potential energy for the three cases at a fixed volume fraction of polymeric depletants (ϕD = 0.1). The association energy of the perfectly matched TC pair with RT = RC = 2.8 is Ea ≅ 19, and that with RT = RC = 1.5 is Ea ≅ 3. Evidently, the depletion effect of the cavity with the size comparable to solvent particle is insignificant. However, it becomes more substantial as the cavity size grows. When the target is smaller than the cavity, depletion still appears and Ea ≅ 6 for the mismatched pair with RT = 2.0 and RC = 2.8. Ea of the mismatched TC pair is less than that of the perfectly matched pair because the extent of overlap between the T/C excluded volumes is less. Thus, the attractive depletion is more effective in the nanoparticle-imprinting system with larger cavities. It is anticipated that the association tendency of the perfectly matched TC pair is stronger than that of the mismatched one. B. Effect of Depletant Concentration on Association Fraction. When the depletion attraction is important, the uptake of targets by the imprinted materials depends on the depletant concentration. The extent of the uptake can be expressed in terms of the association fraction (θa) which is defined as the ratio of the average amount of adsorbed complex TC to the total number of the cavities. Consider a dispersion of nanoparticles whose number equals that of cavities, mixed with polymeric depletants. In this work, an adsorbed complex TC is formed if the separation (H) between nanoparticle and cavity on imprinted material is less than 0.2. The equilibrium adsorption amount of nanoparticles is acquired for different depletant concentrations (ϕD). Figure 3 shows the variation of θa with ϕD for different sizes of nanoparticles and cavities. In general, θa can be raised by depletant addition, and it seems to reach a plateau at high enough ϕD, regardless of the depletant size such as osmolytes (LD = 3) or polymers (LD = 12). In the presence of nonadsorbing polymers, the formation of perfectly matched TC pairs is insignificant for small radius (RC = RT = 1.5), as depicted in Figure 3a. However, the target-substrate adsorption becomes substantial and relies highly on ϕD for large radius (RC = RT = 2.8). On the other hand, as illustrated in Figure 3b, in the mismatched TC system (RC = 2.8 and RT = 2.0), the uptake is still enhanced by the addition of polymeric depletants, but the increment of θa with ϕD is evidently less than that of perfectly matched TC system. To explain the simulation results, the association energies (Ea) of those systems at different depletant concentrations are also evaluated. Figure 4 shows that Ea rises with increasing ϕD. That is, significant adsorption of the target onto the substrate is basically driven by strong attractive depletion associated with sufficiently high depletant concentration, as demonstrated in the case of perfectly matched TC pair with large radius. In contrast, Ea is too small to function for TC pair with small radius. Nonetheless, for mismatched TC pair with large cavity, the influence of the depletant concentration on the association energy is significant enough to cause adsorption. It is interesting to find that although Ea continues ascending, θa reaches a plateau upon the increase of the depletant concentration. This result suggests that the entropy loss associated with adsorbed targets becomes important at high ϕD. C. Selective Recognition. In experiments of molecular recognition, targets with shape complementarity are preferred more than incorrectly shaped molecules that do not fit the binding sites. Similarly, in experiments of selective recognition of nanoparticles, targets matched to the cavities perfectly are
Figure 3. (a) Variation of the association fraction (θa) with the depletant concentration (ϕD) for large (R = 2.8) and small (R = 1.5) TC pairs. Depletants with different lengths (LD = 3 and LD = 12) are examined. (b) Variation of the association fraction (θa) with the depletant concentration (ϕD) for perfectly matched and mismatched TC pairs at LD = 12.
Figure 4. Variation of the association energy (Ea) between a TC pair with depletant concentration (ϕD) for a large perfectly matched TC pair, a small perfectly matched TC pair, and mismatched TC pair at LD = 12.
favored more than nanoparticles mismatched to the binding sites. Since the formation of mismatched TC pairs has been shown to be probable based on depletion attraction only, the 19874
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Figure 5. (a) Variation of the competitive association fraction (θa) with the depletant concentration (ϕD) for perfectly matched and mismatched TC pairs at LD = 12. (b) Variation of the selectivity with the depletant concentration (ϕD) at LD = 12. The selectivity is defined as the association fraction ratio of perfectly matched TC pairs to mismatched TC pairs. (c) Variation of the number of the adsorbed targets with time for perfectly matched TC pairs (RC = 2.8 and RT = 2.8) and mismatched TC pairs (RC = 2.8 and RT = 2.0) at ϕD = 0.2.
Therefore, the optimal ϕD corresponds to strong depletion for perfectly matched pairs but weak adsorption for mismatched ones. Although both association fractions are enhanced by the depletant addition, there exists a significant depression of θa in the plateau regime due to competitive adsorption, according to the comparison between Figures 5a and 3b. It is because the available adsorption sites for large nanoparticles have been effectively reduced by the occupation of small nanoparticles. To understand the mechanism of competitive adsorption, the association kinetics of the two kinds of TC pairs at ϕD = 0.2 is examined. Figure 5c shows the variation of the number of adsorbed nanoparticles with the time steps. In the earlier stage, the mismatched TC pairs are easily formed but the perfectly matched TC pairs seldom occur. As time proceeds, the number of the former reaches its maximum and starts to descend, while that of the latter continues to rise. This result can be attributed to faster migration and easier access to the cavity associated with small nanoparticles. However, their association energy is too weak to resist thermal fluctuations. Consequently, small nanoparticles move out of the cavities eventually. D. Synergetic Effect of Affinity and Depletion. It is known that the adsorption of macromolecules or nanoparticles onto the binding sites on a substrate for recognition is generally attributed to the enthalpic affinity between them, in addition to shape matching (entropic depletion interaction). However, the
competitive adsorption among nanoparticles with different radii is expected to take place. Moreover, the influence of the presence of mismatched TC pairs on the association fraction of perfectly matched TC pairs plays an important role in the shape selectivity of the cavity. In this work, we consider a dipersion consisting of 98 targets (RT = 2.8) and 98 nanoparticles (RT = 2.0) exposed to the imprinted material with 98 cavities (RC = 2.8) under various concentrations of polymeric depletant. Figure 5 demonstrates the equilibrium uptakes of targets perfectly matched to cavities and nanoparticles mismatched to cavities. Both association fractions grow with increasing depletant concentration and reach plateaus at high enough ϕD, as shown in Figure 5a. θa for perfectly matched TC pairs is always greater than that for mismatched TC pairs. These outcomes are similar to those of only one type of target, as illustrated in Figure 3b. The shape selectivity can be defined as the θa ratio of perfectly matched to mismatched TC pairs. Figure 5b shows that the selectivity varies with the depletant concentration nonmonotonically and exhibits a maximum as high as about 40 at ϕD = 0.2. When the depletant concentration is low, the depletion attraction is too weak to result in significant adsorption for both types of nanoparticles. On the contrary, as the depletant concentration is high, the depletion attraction is so strong that significant adsorption occurs for both types. 19875
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association energy (Ea). As depicted in the inset of Figure 6, Ea associated with affinity is increased 2−3 kBT by the depletant addition. Our simulation results demonstrate that the uptake of small nanoparticles with weak enthalpic affinity can be greatly enhanced as the entropic depletion effect appears because of the synergetic effect of depletion and affinity. In fact, the depletion effect associated with shape complementarity between targets and the imprinted material can be of paramount importance to the nanoparticle-imprinting technology.
aforementioned analyses for target recognitions show that the adsorption of large enough nanoparticles to the cavities can be driven by the depletion attraction only. That is, even in the absence of enthalpic affinity, the recognition of large nanoparticles with shape complementarity generated by nanoparticle-imprinting can be enhanced by the addition of polymeric depletants. On the contrary, for small molecules or nanoparticles, depletion attraction is weak and enthalpic affinity between targets and cavities is required to achieve recognition. When the small nanoparticle with weak enthalpic affinity is considered, one may wonder whether the synergetic effect of enthalpic affinity and entropic depletion arises. Their joint efforts may be able to improve the recognition driven by either one of them significantly. To explore the synergetic effect, one examines a system involving perfectly matched TC pairs with a small size (RT = RC = 1.5), as shown in Figure 3a, and with weak enthalpic affinity (aTC = 18−24). Note that aTC = 25 means that no enthalpic affinity exists between targets and cavities. Figure 6
IV. CONCLUSIONS The recognition of nanoparticles by imprinted material is investigated through dissipative particle dynamics simulations. The formation of adsorbed target−cavity (TC) complex is generally believed to be driven mainly by strong enthalpic affinity. However, shape complementarity between target and cavity leads to the emergence of depletion attraction which has an entropy origin. The target−cavity interaction energy U(H) is acquired, and the comparison of U(H) between entropic depletion and enthalpic affinity is made. The energy well of the former is deeper than that of the latter. The association energy Ea can be strengthened by the depletant addition, which suggests that depletion may play an important role in the adsorption of targets onto the imprinted substrates. At a given depletant concentration, the strength of attractive depletion grows with increasing size of a perfectly matched TC pair owing to significant increase in the overlap of their excluded volumes. In addition, the association tendency of the perfectly matched TC pair is stronger than that of the mismatched one. In terms of the association fraction (θa), the uptake of targets can be enhanced by elevating the concentration of polymeric depletant (ϕD). However, θa reaches a plateau at high enough concentrations, indicating that the entropy loss associated with adsorbed targets becomes important at high ϕD. The competitive recognition between perfectly matched and mismatched TC pairs is studied by considering the screening of a mixture of nanoparticles with different radii. A very high selectivity can be achieved at an optimal depletant concentration. The kinetics of selective recognition is also monitored. It is found that small nanoparticles migrate fast and access the cavity easily. Nonetheless, they move out of the cavities eventually owing to their weak association energy. Finally, the synergetic effect of entropic depletion and enthalpic affinity is proved to enhance the association fraction substantially for small perfectly matched TC pairs with weak enthalpic affinity. Our simulation results reveal that the depletion effect associated with shape complementarity between targets and the imprinted material can be of paramount importance to the nanoparticle-imprinting technology.
Figure 6. Variation of the association fraction (θa) with the interaction parameter between the target and the cavity (aTC) for ϕD = 0 and ϕD = 0.3 with LD = 12. In the inset, variation of the association energy (Ea) with aTC at the same conditions.
demonstrates that the association fraction (θaff a ) is quite small (less than 2%) as the depletant is absent (ϕD = 0%) and the interaction parameter (aTC) exceeds 20. Note that affinity grows with lowering aTC and enthalpic affinity becomes significant for aTC = 18, by which θaff a is about 10%. When the depletant is added (ϕD = 30%), the depletion attraction comes into play and the uptake of target (θaaff+dep) is significantly increased. For example, the association fraction is aff+dep raised from 1% (θaff ) for aTC = 21 and from 2% a ) to 7% (θa aff aff+dep (θa ) to 11% (θa ) for aTC = 20. The joint efforts of entropic depletion and enthalpic affinity can be manifested by examining the case of aTC = 25 in which enthalpic affinity is absent and θdep a is still less than 2% after the depletant addition. As one can dep see, θaff+dep > θaff a a + θa , that is, the uptake of target is greater than the combing contributions from depletion and affinity only. Even for significant affinity (aTC = 18), the synergetic aff+dep effect is strong and θa ascends from 10% (θaff ). a ) to 32% (θa aff+dep aff In other words, the difference between θa and θa increases as affinity (aTC) grows. The synergetic effect of entropic depletion and enthalpic affinity can also be realized from the
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research work is supported by Ministry of Science and Technology of Taiwan. Computing times provided by the 19876
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National Taiwan University Computer and Information Networking Center are gratefully acknowledged.
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DOI: 10.1021/acs.jpcc.6b07260 J. Phys. Chem. C 2016, 120, 19871−19877
