Screening of nanoparticle properties

Abstract

A nanoparticle screening chip and a method using said chip allowing for determining physical properties of nanoparticles, wherein the screening chip comprises a substrate having a working surface divided into a plurality of areas, wherein (1) each of these areas presents different surface properties defined by surface energy component (d,b,a), the total free energy .sub.TOT of the surface of each area being defined as follows: .sub.TOT=.sub.LW+2(.sub.+.sub.).sup.0.5, wherein the components are: .sub.LW=dispersive component=d, .sub.+=electron acceptor component=b, .sub.=electron donor component=a; and (2) each of these areas comprises a plurality of subareas, each subarea comprising an array of sub-micrometric holes or elongated grooves with a different aperture size (S1, S2, S3, . . . ).

Claims

1. A method for screening nanoparticles to determine physical properties thereof, the method comprising: (a) feeding a suspension or solution of nanoparticles to the working surface of the nanoparticle screening chip, (b) incubating the suspension or solution of nanoparticles on the working surface for an incubation time (t) and thereafter rinsing the nanoparticle screening chip, and (c) determining physical properties of the nanoparticles by analyzing the nanoparticle screening chip by optical microscopy.

2. The method according to claim 1, wherein the incubation time (t) is between 2 seconds and 60 minutes.

3. The method according to claim 1, wherein: the (a) feeding the suspension or solution of nanoparticles to the working surface further comprises adjusting the pH and/or salt concentration of the suspension or solution of nanoparticles; and/or the (b) incubating the suspension or solution of nanoparticles on the working surface further comprises adjusting the pH and/or salt concentration of the suspension or solution of nanoparticles.

4. The method according to claim 1, wherein the determining the physical properties of the nanoparticles further comprises calculating the surface properties of the nanoparticles by determining a surface free energy balance of an acid-base free energy component (G.sup.AB.sub.adh) and a Lifshitz-Van der Waals surface energy component (G.sup.LW.sub.adh), wherein G.sup.AB.sub.adh is defined by Equation 2, and G.sup.LW.sub.adh is defined by Equation 3:
G.sub.adh.sup.AB=2 ({square root over (.sub.n.sup.AB)}{square root over (.sub.l.sup.AB)})({square root over (.sub.s.sup.AB)}{square root over (.sub.l.sup.AB)})Equation2
G.sub.adh.sup.LW=2 ({square root over (.sub.n.sup.LW)}{square root over (.sub.l.sup.LW)})({square root over (.sub.s.sup.LW)}{square root over (.sub.l.sup.LW)})Equation 3 wherein, in Equations 2 and 3: AB represents the acid-base interaction; .sub.n(AB), .sub.l(AB) and .sub.s(AB) are the acid base components of the surface energies of the nanoparticle (n), a medium of the suspension or solution (l) and the solid surface (s) of the area, respectively; and .sub.n(LW), .sub.l(LW) and .sub.s(LW) are the Lifshitz-Van der Waals components of the surface energies of the nanoparticle (n), the medium of the suspension or solution (l), and the solid surface (s) of the area, respectively.

5. The method according to claim 1, wherein the (c) determining the physical properties of the nanoparticles comprises inserting the nanoparticle screening chip in a reading device configured to optically determine the presence and location of nanoparticles within the areas and subareas of the working surface, wherein the reading device comprises a light source, a holding port configured to hold the nanoparticle screening chip in front of the light source, and a microscope configured to measure the reflection image of the working surface.

6. The method according to claim 1, wherein the incubation time (t) is between 5 seconds and 45 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

(2) FIG. 1 is a schematic perspective view of an embodiment of a nanoparticle screening chip;

(3) FIG. 2 is a schematic section across a further embodiment of a nanoparticle screening chip during feeding with a nanoparticle containing fluid;

(4) FIG. 3 is a schematic representation of a further embodiment of a nanoparticle screening chip showing a plurality of areas (noted A01, . . . in FIG. 3(A)) with different dispersive, Acid-Base and charge properties, and within each of said areas are located subareas with different hole sizes (shown as three rectangles inside areas A01, A02 and A03, although not represented, each other of the areas preferably have the same or equivalent subareas);

(5) FIG. 4 is a schematic representation of unfavorable conditions preventing hydrophobic nanoparticles to bind to a hydrophobic surface, as well as a way to make these conditions more favorable.

(6) FIG. 5 is a diagram showing the kinetics of the binding of nanoparticles to a surface, the slope s (straight line) of the curve being proportional to the surface free energy balance G.sub.adh.

(7) FIG. 6 are two SEM photographs of 200 nm nanoparticles, once on a substrate having grooves of 300 nm aperture (A) and once on a substrate having grooves of 100 nm aperture (B).

(8) FIG. 7 is a schematic setup of a reading device useable for implementing a method as described herein in particular with a nanoparticle screening chip of the invention.

(9) Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings.

DETAILED DESCRIPTION

(10) As depicted in FIG. 1-3, the method preferably uses a screening chip 1 (also called sensing chip) comprising an optically reflective substrate 10 with a (working) surface 11 provided with areas 12 having different surface properties.

(11) As schematically represented in FIGS. 1 and 3, each area 12 is characterized by given values of the surface free energy components, the Total Free Energy of a solid surface being defined as:
.sub.TOT=.sub.LW+.sub.AB=.sub.LW+2(.sub.+.sub.).sup.0.5

(12) wherein the components are:

(13) .sub.LW=dispersive (Lifshitz-van der Waals) component=d

(14) .sub.+=electron acceptor component=b

(15) .sub.=electron donor component=a

(16) Each single area 12 of the surface can thus be defined by the three components (also called triplet) (d,b,a). The surface 11 of the sensing chip 1 can thus be characterized by a plurality of areas 12 and each of them presents different surface properties i.e. surface energy component (d,b,a).

(17) Hence, NMs 20 in suspension or in a colloidal solution are characterized by the equivalent triplets of values of the surface energy according to their surface properties.

(18) So, a triplet (d,b,a) can be defined for any given NM, (d,b,a)NMs.

(19) As shown in FIG. 2, NMs 20 flowing (as indicated by horizontal arrow 30) along the sensing chip's surface 11 by means e.g. of a liquid cell will be attracted (illustrated by arrows pointing to surface 11) by the respective areas A=A(d,b,a) according to their surface properties (d,b,a)NMs. NMs having different surface properties will not be attracted or even be repulsed (arrows pointing away from surface 11) and thus stay within the liquid until they reach an area corresponding to their surface properties (d,b,a)NMs.

(20) By this method, the (d,b,a) triplet of the NMs will be reconstructed according to the affinity of the NMs to the different areas with the corresponding surface properties.

(21) Furthermore, each area 12 on the sensor chip is composed of different smaller areas (subareas 13) as shown in FIG. 3 (B) characterized by an array of sub-micrometric holes or grooves 15. Each subarea 13 is characterized by holes or grooves 15 with different aperture sizes, S, such as S1, S2, S3, . . . , Sn.

(22) Each subarea 13 is characterized by a particular optical response, when illuminated by a beam of visible light at a certain angle of incidence and polarization. As a result of the nanograting, the incident light is diffracted at a given wavelength as a function of the refractive index of the hole or groove 15 containing or not the NMs. The presence of NMs 20 in the holes or grooves 15 can be thus monitored by measuring the wavelength of the diffracted light.

(23) Hence, NMs 20 during their transport along the surface 11 of the chip 1 are attracted (or not) by hydrophobic or other forces towards the surface. Among the attracted NMs, only those with a diameter smaller than the aperture size of the holes S will be captured within the holes or grooves 15. NMs captured within the holes will locally change the refractive index of the nanoholes and modify the optical response of the grating. By monitoring the optical response, the presence of NMs with a diameter smaller than a certain size S.sub.min (hole size) are detectable.

(24) The optical reader preferably consists of a microscope in dark field (DF) configuration, in particular with a CCD camera enabling to capture and to measure the reflection image of the whole sensing surface.

(25) The optical reader is characterized by a certain Field of View (FoV) and a Numerical Aperture (NA). The NA determines the angle of incidence of the detection light beam. For a given NA, flat areas (non-structured, without diffraction grating) will not reflect light and will appear black to the detector. On the contrary, areas with the diffraction grating will be reflected at a given wavelength, function of the geometrical parameter of the grating and the refractive index of the holes (with or without presence of NMs).

(26) When NMs fill the holes of the grating the reflected color of the area will change accordingly, so the presence of NMs in the holes can be by monitored by color changes visually or with a camera sensor.

(27) In another aspect, the invention in particular relates to the sensing chip (nanoparticle screening chip) having the features as described herein.

(28) According to the inventors, the main advantage of the invention is that the proposed solution enables the rapid screening of samples containing NMs. Indeed, the sensor will enable the detection within a few seconds (and with very limited sample amount, such as few hundreds of l) the presence of NMs smaller than size S.sub.min. Furthermore, the use of different S.sub.min within one subarea and hence the presence and proportion of NMs within one or more of the differently sized holes will provide information about NM size distribution. Together with the minimum size, the device will allow to characterize the surface property of the NMs, determining their acidic, basic or dispersive components, as well as their sizes and size distributions.

(29) Below are described some experiments and considerations done in the context of making the present invention. These experiments provide further guidance for the skilled person in order to reduce to practice the present invention. The information below should however not be construed as limiting the invention to the particular embodiments and results described.

(30) Experiments and Experimental Setup

(31) A) Modification of the Surface Energy Components

(32) A silicon substrate (it might be glass or any other flat surface) was modified by different plasma deposition in order to tune the surface hydrophobicity. Polytetrafluoroethylene was used to generate a hydrophobic surface, the deposition was realized using pure octofluorocyclobutane (C.sub.4F.sub.8) as the gas precursor at a pressure of 3.2 Pa (27 mTorr), applying a power of 142 W for 5 min. Plasma-polymerized acrylic acid was deposited as a hydrophilic surface, using acrylic acid as the gas precursor at a pressure of 2.1 Pa (16 mTorr), applying a bias power of 400 W for 5 min.

(33) Polyelectrolyte Layer by Layer Deposition

(34) In order to tune the surface hydrophobicity, a layer-by-layer deposition of two polyelectrolytes was realized. The plasma-modified substrates were incubated for 2 min in Poly(diallyldimethylammonium chloride) (PDDA) 2% solution in water or in Poly(sodium 4-styrene sulfonate) (PSS) 2% in water for the self-assembly deposition of each polyelectrolyte layer-by-layer, starting from PDDA (positively charged) and alternating with PSS (negatively charged). After each step, the substrate was rinsed with milliQ water and dried under nitrogen flow.

(35) A first study was realized using 200 nm diameter polystyrene particles (Polybead microspheres, Polysciences). Those commercial particles were chosen as a model because of the large range of sizes and surface functionalization (corresponding to different hydrophobicity and charges) offered by the provider. The non-modified particles are stabilized by sulfonate groups; they are negatively charged and hydrophobic.

(36) The modified surfaces were incubated with the model particles in order to evaluate the binding, associated to the interaction forces between the particles and the surfaces. The experiment was realized using 16 different conditions of salt concentration ([NaCl]=0/1/10/100 mM) and pH (2/4/7/10) in aqueous solution in which the particles were dispersed at the original concentration. The incubation took place with the substrate fully immersed in the different solutions for 30 min, then rinsed thoroughly with milliQ water and dried under nitrogen flow. The surfaces were finally imaged with Scanning Electron Microscopy (SEM).

(37) The same experiment was realized with polystyrene particles modified with hydroxyl groups. This surface modification conferred to the particles a higher hydrophilicity.

(38) In order to study the selective binding of NPs on chemically modified surfaces, two set of samples have been prepared. A first set of Silicon samples had been coated first with a plasma deposited layer of PTFE and then several layers of polyelectrolyte (PPS/PDDA) for decreasing the hydrophobicity level. A second set of sample has been prepared with a starting layer of PAA. Theses samples have been modified as well with PE deposition to decrease the surface hydrophobicity.

(39) The results of characterization are presented in Tables 1 and 2.

(40) TABLE-US-00001 TABLE 1 Summary of the characterization of the PTFE modified surface Contact AFM angle Ellipsometry Roughness Z-potential () Height (nm) (nm) at pH 7 PSS #3 19.8 0.4 0.65 0.01 0.76 0.08 57.64 0.31 PDDA #3 37.4 0.3 0.44 0.01 0.85 0.09 4.13 0.33 PSS #2 53.3 0.8 0.69 0.01 0.83 0.08 62.37 0.59 PDDA #2 65.4 0.6 1.11 0.02 0.85 0.09 4.93 0.25 PSS #1 64.6 0.7 0.66 0.01 0.48 0.05 60.22 0.90 PDDA #1 79.3 0.7 1.43 0.02 0.45 0.05 26.28 0.24 PTFE 106.8 0.4 0.29 0.03 61.18 0.13

(41) TABLE-US-00002 TABLE 2 Summary of the characterization of the PAA modified surface Contact AFM angle Ellipsometry Roughness Z-potential () Height (nm) (nm) at pH 7 PSS #3 22.8 0.4 0.65 0.01 1.87 0.19 62.60 0.33 PDDA #3 28.2 0.6 0.44 0.01 1.88 0.19 6.53 0.68 PSS #2 25.2 0.1 0.69 0.01 1.09 0.11 50.37 0.92 PDDA #2 30.9 0.4 1.11 0.02 0.77 0.08 2.80 0.15 PSS #1 41.4 0.8 0.66 0.01 0.65 0.07 47.36 0.29 PDDA #1 52.1 0.5 1.43 0.02 0.12 0.01 5.04 0.53 PAA 58.3 0.5 0.23 0.02 78.12 1.38

(42) As shown in Table 1, the sample coated with PTFE plasma-deposited layer was highly hydrophobic, with a contact angle of 106. For each polyelectrolyte layer the contact angle measurement showed a decrease in the hydrophobicity of the surface, from 80 (hydrophobic) for the first layer, to 20 (highly hydrophilic) for the 6.sup.th layer. The ellipsometry (optical technique for investigating the dielectric properties of thin films) enabled to measure the thickness of each polyelectrolyte layer. The PSS layers were around 0.7 nm thick, and each PDDA between 1.4 and 0.4 nm, the first one being the thickest. The AFM provided information on roughness that was increasing with the first 3 layers from 0.3 to 0.8 before being stable around 0.8 nm for the last 3 layers. The z-potential was measured for different pH, for all layers a negative z-potential was obtained for the whole range of pH, especially for the PTFE non-modified and the PSS layers, and closer to neutral for the different PDDA layers. This result can be explained knowing that the PDDA is positively charged and the PDDA and PTFE negatively charged.

(43) The experiments performed on the PAA modified surface (Table 2) showed the same trend. With a base layer of PAA more hydrophilic, one could indeed reach the same surface properties with an increase in hydrophilicity with the polyelectrolytes layers, a more important increase in roughness and a zeta-potential negative for all conditions. The 6.sup.th layer enabled to obtain close surface properties with two substrates of different properties, PTFE or PAA.

(44) The XPS and ToF-SIMS analysis of the surface modifications by PTFE plasma deposition and layer-by-layer polyelectrolytes deposition was also performed (data not shown). The surface analysis through XPS experiments demonstrated the presence of the PTFE on the silicon substrate, and the ToF-SIMS experiment confirmed those observations, with an analysis that is more surface sensitive, the coverage of the silicon substrate with the PTFE plasma, and of the PTFE base layer with the 6 layers of polyelectrolytes.

(45) Nanoparticles Binding Study

(46) Hydrophobic Nanoparticles

(47) Considering that the silicon substrates modified by plasma deposition of PTFE shows a high hydrophobicity, a first experiment was performed with hydrophobic particles in different conditions. The PS particles in 16 conditions of pH and ionic strength were incubated on the surface in order to evaluate the binding of hydrophobic particles with a hydrophobic surface, expected to be high because of the hydrophobic forces. The surfaces were then analyzed with SEM and the ratio of the surface coverage was calculated for the different conditions using ImageJ software. Those results are presented in Table 3.

(48) TABLE-US-00003 TABLE 3 Surface coverage (%) of PS particles on PTFE, contact angle = 105 pH 2 4 7 10 Z-potential 3 32 54 140 [NaCl] 0 2.5 2.4 0.8 0.6 mM 1 2.1 0.2 0.1 0.9 10 5.6 6.6 0.1 0.0 100 2.5 27.0 2.8 0.3

(49) Surprisingly, the binding rate was low for all conditions, with a slight trend to a higher binding for low pH and high salt concentration.

(50) The low binding of NP on the hydrophobic PTFE surface was hypothetically attributed to the poor wettability of the PTFE surface with the creation of micro-bubbles, which impede the contact between the surface and the particles in the water suspension, preventing the hydrophobic forces to take place.

(51) The negative charges at the surface of the PTFE layer and of the particles would indeed be decreased for low pH and high ionic strength, in those conditions the long range repulsion by electrostatic forces would then be drastically decreased, enabling the shorter range hydrophobic forces to take place.

(52) The following hypothesis is proposed to understand the low binding for all conditions. The interface between the surface and the particles should be considered as multiple interfaces: since the particles are incubated in an aqueous solution, the water is playing an important role into the substrate-particles interaction with a substrate-water interface and another interface particles-water. The exposition of the highly hydrophobic substrate to water would generate micro-air bubbles to limit the contact, and the same would happen on the surface of the hydrophobic particles. Those micro-bubbles could create a physical barrier, preventing the particles to approach to the surface close enough for the hydrophobic interactions to take place. To limit the presence of those air bubbles enabling the water contact on the surface, the hydrophobicity should then be decreased.

(53) In order to verify this hypothesis, the same experiments were performed with the PTFE surface modified with polyelectrolyte layers. Since the number of polyelectrolyte layers has a direct influence on the hydrophobicity (as seen with the contact angle measurements), it was possible to achieve four different degree of hydrophobicity, corresponding to contact angles of 105, 70, 50 and 20. The observation of the surfaces after incubation of the particles showed a progressive increase of the binding, with a more and more important surface coverage of the surface for a decreasing contact angle. The calculated surface coverage by the particles for the lowest contact angle is presented in Table 4.

(54) TABLE-US-00004 TABLE 4 Surface coverage of PS particles on PTFE + PE (6 layers), contact angle = 20 pH 2 4 7 10 Z-potential 3 49 60 100 [NaCl] 0 54.2 57.2 33.0 3.6 mM 1 40.4 37.8 30.5 30.9 10 49.2 43.4 48.0 47.6 100 83.3 64.3 55.1 51.5

(55) Compared to the results without polyelectrolytes, the surface coverage by the particles is dramatically increased with the 6 polyelectrolytes layers. Moreover, the trend already observed is confirmed, with an important increase of the binding with the decrease in pH and the increase of ionic strength. To compare the binding rate obtained on a hydrophobic substrate+hydrophilic superficial layer with the binding rate on a hydrophilic substrate and to verify the theory of the long range interactions, another experiment was performed using the previously described plasma deposited PAA, with and without the polyelectrolyte modification. The degree of hydrophobicity was tuned from around 60 of contact angle without polyelectrolyte to 20 with six layers of polyelectrolytes. The different surfaces after incubation of hydrophobic polystyrene particles in the same conditions as before were observed by SEM and the results in terms of surface coverage is presented in Table 5 for PAA alone (a) and PAA with six layers of polyelectrolytes (b).

(56) TABLE-US-00005 TABLE 5 Surface coverage of PS particles on a. PAA, b. PAA + PE pH 2 4 7 10 a. PAA Z-potential 26 45 78 83 [NaCl] 0 0.0 3.4 0.0 0.0 mM 1 0.9 0.1 0.0 0.0 10 0.1 0.2 1.8 0.0 100 80.8 51.6 39.4 0.4 b. PAA + PE Z-potential 20 50 63 53 [NaCl] 0 1.0 0.0 0.0 0.0 mM 1 43.1 0.0 0.0 0.0 10 32.0 0.6 0.0 0.1 100 59.3 54.2 19.9 27.5

(57) As can be observed for both PAA alone and PAA+PE, the surface coverage is extremely low for most of the conditions, with salt concentration between 0 and 10, and all pH on PAA alone and pH 4 to 10 on PAA+PE. The trend already observed before is still present with an increase of the binding rate with the increase of ionic strength and the decrease of pH, but even more than before since the particles binding increases only in those extreme conditions. Comparing the results on the PAA and PTFE substrates without polyelectrolytes (Table 3 and 5a), it can be assumed that, with the highest ionic strength, the binding is more important on PAA than on PTFE because of the physical barrier existing because of the highly hydrophobic properties of the PTFE. But, the main difference appears to be between PTFE+PE and PAA+PE (Table 4 and 5b). Indeed, adding the polyelectrolytes layers induce a large increase in the binding on the hydrophobic substrate, whereas the change with polyelectrolyte on PAA is significant only for pH 2 with salt, showing that the hydrophilic superficial layer only permit the hydrophobic interactions to take place, resulting in the binding of the hydrophobic particles only on the hydrophobic substrate.

(58) A schematic interaction model of the long-range hydrophobic forces and the effect of the hydrophobicity of the substrate is presented in FIG. 4.

(59) As presented in FIG. 4 showing the theoretical model of interactions according to different conditions, in the first tested case, the hydrophobic PTFE was in direct contact with the aqueous medium containing the particles. In this condition an air interface is generated to limit the contacts between water and the hydrophilic substrate, and this prevents the physical interaction between the particles and the substrate. In addition, if the electrostatic repulsion is not avoided through a decrease of pH and/or an increase of ionic strength, the hydrophobic interactions acting at shorter range can't take place. On the contrary, by minimizing the electrostatic repulsion thanks to low pH and high salt concentration, and enabling the physical interaction through the use of a superficial hydrophilic layer in top of the hydrophobic substrate, the most favorable conditions can be obtained for the binding of hydrophobic particles on the hydrophobic substrate. The hydrophilic layer has to be thin enough (<<100 nm) in order for the hydrophobic interactions to take place at the long range. The use of a polyelectrolyte layer is in this case a good way to obtain a high hydrophilicity with a thin coverage of the hydrophobic substrate. The superficial hydrophobicity/hydrophilicity degree, also called wettability, is therefore not driving the binding but enables the hydrophobic interactions to take place.

(60) Hydrophilic Particles

(61) The second part of the study consisted in the evaluation of the binding of hydrophilic particles on the previously used substrates. Considering that the system mechanism is based on hydrophobic interactions, its interest would be to enable to characterize the particles hydrophobicity thanks to different binding response on the patterned areas. The particles used for this part of the study were the same polystyrene particles but modified with hydroxyl groups which give them hydrophilic properties.

(62) The PSOH particles in 16 conditions of pH and ionic strength were incubated on the different surfaces. The analysis by SEM enabled then to calculate the ratio of the surface coverage for the different conditions. The results obtained on PTFE, PTFE+PE, PAA and PAA+PE are presented in Table 6.

(63) TABLE-US-00006 TABLE 6 Surface coverage of PS-OH particles on a. PTFE, b. PTFE + PE, c. PAA, d. PAA + PE pH 2 4 7 10 a. PTFE [NaCl] 0 0.0 0.2 0.0 0.0 mM 1 0.0 0.0 0.0 0.0 10 6.9 0.0 0.0 0.0 100 0.0 0.6 4.0 0.0 b. PTFE + PE [NaCl] 0 0.3 0.2 0.0 0.0 mM 1 0.9 0.4 0.0 0.2 10 0.6 0.7 0.1 0.1 100 2.6 10.2 1.9 6.5 c. PAA [NaCl] 0 0.2 0.0 0.0 0.0 mM 1 0.8 0.5 0.0 0.0 10 0.2 0.1 0.1 0.0 100 36.5 51.7 22.2 1.6 d. PAA + PE [NaCl] 0 0.3 0.0 0.0 0.0 mM 1 0.3 0.0 0.0 0.0 10 2.3 0.1 0.0 0.0 100 54.0 37.8 1.0 1.4

(64) As can be seen, the hydrophilic particles have a binding rate extremely low (<1% in most of the cases) for all conditions on the hydrophobic substrate, modified or not. On the hydrophilic substrate, modified with the polyelectrolytes or not, there is no binding in most of the conditions, with a significant binding only for high salt concentration and negative pH as already observed before. Those results confirm what was expected, with no hydrophobic interactions the hydrophilic particles did not bind to the different surfaces.

(65) B) Method for the Determination of the Surface Free Energy of the (Unknown) Nanoparticles

(66) The generic method for the determination of the surface energy of an unknown sample of nanoparticles is similar to the extended DLVO model (Van Oss et al., J. Colloid Interface Sci. 111, 378-390).

(67) When a nanoparticle in solution (many nanoparticles) gets in close contact with a surface there are different forces attracting or repelling the NP to/from the surface

(68) The forces are according
F.sub.adh=F.sup.LW+F.sup.EL+F.sup.AB
With F.sup.LW: Lifshitz-Van der Waals interaction force (attractive, short range)

(69) F.sup.EL: Electrostatic interaction force (attractive or repulsive, long range)

(70) F.sup.AB: Acid-base interaction force (hydrophobic interaction)

(71) The Lifshitz-Van der Waals component is always attractive and always present.

(72) The Electrostatic component can be attractive or repulsive, but anyway can be totally screened by the increase of the salt concentration. In any case, its values are known from the measurement of the Zeta potentials of the nanoparticles and the chip active surfaces. Between two surfaces with the same charge they are repulsive.

(73) The AB forces include the hydrophobic interaction.

(74) If one assumes that the attraction forces are ONLY the AB forces, they are long-range (acting when the NP is at several nm from the surface) and they are strong (this is controlled by the present experimental conditions).

(75) The NPs are attracted to the surface and they stay there only if they are in a situation that minimizes the surface free energy of the system surface-liquid-NP. By definition:

(76) l: liquid n: nanoparticle s: substrate

(77) Interfacial Free Energies:

(78) .sub.sl KNOWN

(79) .sub.sn MEASURED

(80) .sub.nl TO BE DETERMINED

(81) Free energy of adhesion G.sub.adh=.sub.nl.sub.sn.sub.sl

(82) The surface free energy balance (G.sub.adh) is the balance between the interfacial energies (of the NP and the surface) with the liquid and the interface of the NP and the surface.

(83) The present goal is to calculate the .sub.nl (the surface energy component of the NP, in particular the LW-hydrophobic component).

(84) The geometric relationship between the surface components can be expressed as follows:
G.sub.adh.sup.AB=2({square root over (.sub.n.sup.AB)}{square root over (.sub.l.sup.AB)})({square root over (.sub.s.sup.AB)}{square root over (.sub.l.sup.AB)})
G.sub.adh.sup.LW=2({square root over (.sub.n.sup.LW)}{square root over (.sub.l.sup.LW)})({square root over (.sub.s.sup.LW)}{square root over (.sub.l.sup.LW)})
wherein AB (resp. LW) represents the acidbase (resp. Lifshitz-Van der Waals) interaction and .sub.n(AB), .sub.l(AB) and .sub.s(AB) (resp. Y.sub.n(LW), Y.sub.l(LW) and Y.sub.s(LW) are the acid base (resp. Lifshitz-Van der Waals) components of the surface energies of the nanoparticle (n), solution/suspension medium (l) and solid surface (s) of the area. .sub.l(AB) is known from the literature and .sub.s(AB) is known for each modified surface (and it is measured and known for each area of the device). .sub.n(AB) is the unknown parameter from the NP to be determined by the present method.

(85) With the presented system, the G.sub.adh can be measured directly by measuring the kinetics of adsorption the nanoparticles on the surface. In particular G.sub.adh is proportional to the slope s of the kinetics of adsorption (see FIG. 5).

(86) Hence, a preferred determination step of a method of the invention could be summarized as follows: (a) One measures and one knows the surface properties of each surface (b) One measures the kinetics of adsorption of the NP on each surface (c) One calculates the surface properties of the NP.

(87) C) Microfabrication of the Areas with Different and Controlled Surface Properties

(88) The areas characterized by controlled surface properties (as explained above) can be micro-patterned on a same surface e.g. by the following microfluidic device (MFD) designed by the inventors. 1) The surface of the chip can be modified by PTFE or PAA or any other polymer with controlled hydrophobicity/hydrophilicity, 2) The MFD is applied on top of the chip, 3) In each area of the MFD is flown as certain number of polyelectrolytes layers in order to locally modify the surface properties.

(89) D) Nanofabrication of the Physical Filter or Capturing Features

(90) The physical filters or capturing structures for the nanoparticles (holes arrays or grooves arrays) are nanofabricated on the surface of the chip e.g. by Ion Beam Milling, but the same structure can be made using different fabrication techniques.

(91) The basic purpose is: 1) to create a diffraction grating (made of holes or grooves) 2) The same diffraction grating is filtering the NP 3) Only the NP smaller than the grating size can fall inside the grating 4) The optical response of the diffraction grating changes only when NP are located inside the grooves 5) In this way, particles smaller than a chosen size can easily be detected.

(92) For example:

(93) Grating 1 which has lines (grooves) of 300 nm wide is fed with 200 nm nanoparticles,

(94) Grating 2 which has lines of 100 nm wide is fed with 200 nm NP,

(95) It is clear that 200 nm NP will fall inside the 300 nm grooves, while they stay on top of the 100 nm grooves (See FIGS. 6 (A) and (B) which are SEM images of Polystyrene NP on a hydrophobic surface) 1) The optical response of the diffraction spectrum of the Grating 1 is modified by the presence of the NP in the grooves (FIG. 6(A)) 2) The optical response of the diffraction spectrum of the Grating 2 is modified by the presence of the NP on top of the grooves (FIG. 6(B))

(96) The changes in the optical response can be following in real time during the NP adsorption and the kinetics curve can be reconstructed to calculate G.sub.adh (see above and FIG. 5). Moreover, it is possible to do a fast screening of the NP size.

(97) E) Illustrative Optical Setup for the Real Time Detection of the NP on the Chip's Surface

(98) A preferred system to analyze the NP on the screening chip basically comprises of a dark field microscope with a large Field-of-view (to detect all the areas of the chip at the same time), which allows detecting NPs inside or outside the diffraction gratings and wherein the detection may be done in real time.

(99) An example is shown in FIG. 7, wherein a light source 41 emits light passing through a first lens system 42, reflected by beam splitter 43 through illumination pupil 44 and a second lens system 45 onto the nanoparticle screening chip 46. The light which is refracted by the screening chip 46 passes through illumination pupil 44, splitter 43 and a third lens system 47 through the aperture 48 of a camera 49.

(100) As an example, such a system may comprise:

(101) from Edmund Optics/Thorlabs:

(102) two Erfle eyepieces 41-347 as lens systems 42 and 45 50 mm beamsplitter cube 32-704 as beam splitter 43 3-4 PCX (or achromatic pairs) lenses and lens mounting system (cage etc.) for lens system 47
Camera 49 may be a CMOS camera with standard lens (f10 mm) Imaging NA adjustable from the camera lens

(103) As there is a common path for imaging and illumination between screening chip 46 and beam splitter 43, there may be some reflections to the image. In fact, the illumination angle can be adjusted by moving the hole plate in illumination pupil 44.

LEGEND

(104) 1 Nanoparticle screening chip 10 Substrate 11 Working surface 12 Areas 13 Subareas 15 Submicronic hole or groove 20 Nanoparticle(s) 30 Liquid flow 40 Dark field detection setup 41 Light source 42 First lens system 43 Beam splitter 44 Illumination pupil 45 Second lens system 46 Sample/screening chip 47 Third lens system 48 Imaging NA adjustment from camera lens 49 Camera