Sub-nanometer thickness and inter-diffusive measurement using optical etched fiber bragg grating technique

Abstract

The device is capable of monitoring and controlling the layer-by-layer buildup at the molecular level using an etched FBG. Etched FBG enhances the sensitivity of a traditional FBG by increasing the light interaction with the deposited layers. This enables sub-nanometer thickness monitoring during the buildup process, allowing the detailed study of adsorption and desorption processes. the proposed mechanism can be used as an instrument for probing inter-diffusive behaviour of macromolecules, bio-polymers and proteins at sub- nanometer scale.

Description

RSA23P0130(IDF-2732) Sub-nanometer thickness and inter-diffusive measurement using optical etched fiber Bragg grating technique FIELD OF THE INVENTION

[0001] The present invention generally relates to an optical technique for in-situ nano-scale thickness and inter-diffusive measurement, particularly to a method and device for in-situ nano-scale thickness and inter-diffusive measurement using etched fiber Bragg grating. BACKGROUND ART

[0002] The layer-by-layer (LbL) self-assembly of polyelectrolytes, proposed by Decher et al., is a technique to fabricate molecularly thin films, on a wide range of substrates, without using expensive infrastructure. LbL technology involves the deposition of the oppositely charged polymers ions, followed by rinsing with de-ionised water to build up PEM films. PEM deposition is primarily driven by electrostatic interaction, and assembly parameters like pH, ionic strength, molecular weight, dipping, and rinsing time. The fabrication protocol determines the morphological and optical characteristics of the PEM film. The properties of polyelectrolyte multilayers (PEMs) have been analyzed in their dry state by utilizing optical techniques such as Ellipsometry and Surface plasmon spectroscopy. The kinetics of the progressive layer build-up and quantitative analysis of LbL adsorption of various polyelectrolyte pairs underRSA23P0130(IDF-2732) different deposition conditions has been explored using various in-situ analysis techniques like Atomic Force Microscopy (AFM), Quartz Crystal Microbalance (QCM), Ellipsometry, Surface Plasmon Spectroscopy and Optical Waveguide Lightmode Spectroscopy (OWLS). These techniques have been used along with other optical techniques like Confocal microscopy, Fluorescence Recovery after Photobleaching (FRAP), and Fluorescence Resonance Energy Transfer (FRET) and non-optical techniques like X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectroscopy (SIMS) for probing interlayer diffusion. Confocal microscopy is more suitable for thick films and cannot be used for films thickness less than 100 nm. Optical techniques like FRAP and FRET require fluorescent tagging of diffusing species, while the non-optical techniques like XPS and SIMS can alter the native structure of PEM films.

[0003] Fiber Bragg Gratings (FBG) sensors are wavelength-modulation based sensors having a very high sensitivity and sampling rate compared to intensity-modulated sensors. They can be used to detect the thickness deposited over its surface in real-time from the change in Bragg wavelength shift due to the change in the effective refractive index of the sensing element. Etched Fiber Bragg Grating (EFBG) sensors, obtained via chemical etching of FBGs, also operate by detecting the shift in Bragg wavelength, with the added advantage of having aRSA23P0130(IDF-2732) significantly higher sensitivity which is comparable to Surface Plasmon Resonance (SPR)-based techniques. EFBGs can be easily embedded in a microfluidic channel to probe a very low volume of analytes, and are ideal for probing the kinetics of ultra-thin film deposition with a thickness range of 1-2 nm during the initial layers deposition.

[0004] The growth mechanism of weak polyelectrolytes participating in the layer buildup process has been under investigation for a long time. pH-dependent modulation in thickness, surface roughness, wettability, and porosity has been studied extensively. PEM buildup takes place largely due to consecutive alternative deposition of oppositely (positively and negatively) charged polymers due to the surface charge reversal at each deposition step, resulting in a gradual build-up of PEM stack. The LbL assembly of Poly (allylamine hydrochloride) (PAH) / Poly (acrylic acid) (PAA) is due to the formation of water-insoluble complexes of polycations and polyanions electrostatic attractions. It has also been shown that (PAH / PAA) multilayers could be adsorbed as “thick and rough" or “thin and smooth" films just by altering the pH of the polyelectrolyte solution. A simple pH-based modulation also helped in tuning the nanoscale porosity of the films. These are ex-situ analyses, performed on films after the deposition process, thus missing out on critical information about the kinetics of layer deposition. In anotherRSA23P0130(IDF-2732) significant QCM-based in-situ study, pH regimes were identified in which (PAH / PAA) buildup was exponential due to interdiffusion of PEs, and in other pH regime, the growth was linear due to opposition of precursor layers for inter-diffusion of polyelectrolytes. It was also shown in an earlier work that low molecular weight PAA can diffuse “in and out”, resulting in an exponential layer build- up. The buildup mechanism was also observed in real- time for certain biopolymers, namely (poly(L- lysine) / hyaluronan (PLL / HA), in which the mechanism of growth was exponential due to the 'in and out’ diffusion of polyelectrolytes, as elaborated in similar works [See for instance, Picart, C. et al. Molecular basis for the explanation of the exponential growth of polyelectrolyte multilayers. Proc. Natl. Acad. Sci. 99, 12531–12535 (2002); Lavalle, P. et al. Direct Evidence for Vertical Diffusion and Exchange Processes of Polyanions and Polycations in Polyelectrolyte Multilayer Films. Macromolecules 37, 1159–1162 (2004)].

[0005] Ellipsometry is one of the most popular methods for thickness characterization and can be used for sub-nanometer thickness measurements, but requires the optical properties of the bulk film to be known in advance, and the substrate roughness results in uncertainty in the measurement, especially for ultrathin films below 25 nm. QCM and AFM are the popular non-optical techniques for monitoring PEM buildup. Although AFM gives absoluteRSA23P0130(IDF-2732) thickness measurement, it involves measurement based on the height difference between the substrate and the film's surface by creating a step patterning or a ‘tip scratch’. QCM, on the other hand, can provide inaccurate thickness measurement due to changes in mass on account of adhered liquids.

[0006] Therefore, there exists a need in the art to develop and disclose a novel device and method for to monitor individual layer growth and control the film thickness precisely, even at the sub-nanometer scale, by fine-tuning the LbL deposition parameters. OBJECTS OF THE INVENTION

[0007] An apparatus for in-situ measurement of thickness and inter-diffusive behaviour of macromolecules, bio-macromolecules and biological entities at sub-nanometer scale. SUMMARY OF THE INVENTION

[0008] Traditionally inter-diffusive behaviours have been investigated using confocal microscopy only in micron scale films. The optical etched fiber Bragg grating (EFBG) technique is able to precisely quantify the in-situ thickness of the single molecular layer being adsorbed onto substrate at sub-nanometer scale. This is done by Etched Fiber Bragg Grating (EFBG) sensors, obtained via chemicalRSA23P0130(IDF-2732) etching of Fiber Bragg Grating (FBGs), and detecting the shift in Bragg wavelength.

[0009] The invention uses an etched FBG placed in a microfluidic channel and monitors the LbL deposition by monitoring the shift in Bragg wavelength which is dependent on the light interaction taking place at the site of etching. The invention is further capable of investigating molecular layer buildup process at nanoscale, with ability to quantify parameters of layer buildup process window, viz. thickness of a single molecular layer being adsorbed as well as observation on inter-diffusive behavior of molecules participating in LbL buildup mechanism by in-situ measurement of adsorbed thickness at sub- nanometer scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG.1 shows (a) Schematic of the experimental setup of LbL deposition on EFBG. (a)Inset: FEM model for EFBG for generating thickness data from experiments (b) Thickness values (^^)in nanometer (nm) for different values of Bragg wavelength shifts (Δ^^^^) using exponential growth model of the FEM model.

[0011] FIG.2 shows (a) LbL deposition due to coulombic electrostatic interaction between anionic PAA and cationic PAH chains. (b) SEM image of PEM coated EFBG. (c) Schematic of in and out diffusionRSA23P0130(IDF-2732) mechanism for (PAH / PAA)n multilayer. (d) PAH / PAA layer deposition expressed in terms of thickness deposited over the EFBG surface by monitoring the shift in the Bragg wavelength.

[0012] FIG.3 is Growth curve of PAH / PAA layer deposition for pH 5.5 and pH 7.

[0013] FIG.4 are Individual deposition curves at each bilayer of the PAH-PAA system for (a) PAA at pH=5.5, (b) PAH at pH=5.5, (c) PAA at pH=7, (d) PAH at pH=7.

[0014] FIG.5 shows Conventions used for calculations with respect to (a) Bragg wavelength data, (b) simulated thickness values.

[0015] FIG.6 are Growth curves for pH 5.5 and pH 7 system for a set of three experiments each (Error bars indicate the deviation in measurements): (a) Pre-rinse growth curve using Bragg wavelength shift data, (b) Post-rinse growth curve using Bragg wavelength shift data, (c) Pre-rinse adsorbed experimental thickness measurement fitted with COMSOL simulations, (d) Post-Rinse adsorbed experimental thickness measurement fitted with COMSOL simulations. Error bars indicate one standard deviation from the mean value, n=3.

[0016] FIG.7 Desorption curves: (a) Desorption curves of PAH and PAA of pH 5.5 system, (b) Desorption curves of PAH and PAA of pH 7 system, (c) DesorptionRSA23P0130(IDF-2732) curves of PAH and PAA of pH 5.5 system with extracted thickness, (d) Desorption curves of PAH and PAA of pH 7 system with desorbed thickness. Error bars indicate one standard deviation from the mean value, n=3.

[0017] FIG.8 shows Mass Fraction analysis: (a) Mass fraction adsorbed and desorbed in pH 5.5 system, (b) Mass fraction adsorbed and desorbed in pH 7 system. Error bars indicate one standard deviation from the mean value, n=3.

[0018] FIG.9 shows (a) Analysis of kinetics of pH 5.5 and pH 7 system. Error bars indicate one standard deviation from the mean value, n=3. (b) Comparison of limit of detection of different PEM thickness monitoring techniques. DETAILED DESCRIPTION OF THE INVENTION

[0019] It is disclosed here a device and method for precisely measuring the thickness and inter- diffusive using at sub-nanometer scale and analyzing in real time, one or more parameters during a deposition process on a substrate. This is done by Etched Fiber Bragg Grating (EFBG) sensors, obtained via chemical etching of Fiber Bragg Grating (FBGs), and detecting the shift in Bragg wavelength.

[0020] The invention uses an etched FBG placed in a microfluidic channel on a surface and monitors the LbL deposition by monitoring the shift in BraggRSA23P0130(IDF-2732) wavelength which is dependent on the light interaction taking place at the site of etching. Referring to FIG. 1, an embodiment of the invention is a device (100) for measuring sub-nano thickness and monitor the inter-diffusive of any charge based material on any surface. The said device (100) comprises of a fluidic channel (1) on the surface (2). An optical fibre (3) is positioned on the fluidic channel (1). One or more charged layers (5) of one or more material is deposited on the optical fibre (3). A light source (6) is provided for the light to pass through the optic fibre (3). The cladding (8) of optical fiber (3) acts like Etched Fiber Bragg Grating (EFBG) sensors, and the thickness of the deposition layer (5) is detected by an optical interrogator system (7) by detecting the shift in Bragg wavelength.

[0021] The invention is further capable of investigating molecular layer buildup process at nanoscale, with ability to quantify parameters of layer buildup process window, viz. thickness of a single molecular layer being adsorbed as well as observation on inter-diffusive behavior of molecules participating in LbL buildup mechanism by in-situ measurement of adsorbed thickness at sub- nanometer scale.

[0022] The invention can measure the thickness of the single molecular layer being adsorbed onto the fiber in real-time (in-Situ). Further, the instantRSA23P0130(IDF-2732) invention could also be a very useful instrument for studying the inter-diffusive behavior of macromolecules, including bio-macromolecules. EXAMPLE

[0023] The invention is further described by means of an example. The example merely illustrates the exemplary embodiments of the invention. It should not be construed to limit the scope of the invention.

[0024] In the instant example, a device and method for, in-situ analysis of weak Poly (allylamine hydrochloride) (PAH) / Poly (acrylic acid) (PAA) polyelectrolyte multilayers (PEM) combination build up has been done using EFBG based optical sensing interrogation system.

[0025] With the aid of PAH / PAA LbL deposition it is demonstrated herein the sub-nanometer monitoring capability of EFBGs. The present invention also includes design of a numerical model which relates Bragg wavelength shift to the thickness deposited over the EFBG. Through accurate in-situ thickness monitoring, our invention presents an opportunity to study the intricacies of LbL deposition even during initial layer buildup.

[0026] An investigation of the adsorption and desorption kinetics of (PAH / PAA) in the layer buildup was done under two different pH conditionsRSA23P0130(IDF-2732) of i.e., pH 5.5 and pH 7. All the experiments were performed by using low molecular weight PAH (Mw ~ 15 kDa) and high molecular weight PAA (Mw ~ 150kDa). As pH plays a major role in the charge density of weak polyelectrolytes, its significance in determining the thickness of the PEM film has been validated through EFBG. Optical EFBG technique allows us to observe the difference in thicknesses at the two pH values, even at the initial stages of layer build-up when the thickness of the film is below 10 nm. This in-situ monitoring also allows a unique perspective on the desorption phenomenon during the rinsing step. PAA is observed to have a linear rate of desorption, while PAH is observed to have an exponential rate of desorption after the rinsing steps. This validates the “in and out” diffusion of low molecular weight PAH, leading to the exponential growth of (PAH / PAA) PEM system. In addition, a quantitative analysis of adsorbed and desorbed polymer masses was also performed to understand the contribution of each step of poly- ions during the multi-layer buildup process. Thus, Bragg wavelength shifts from the EFBG measurements can be used to monitor individual layer growth and control the film thickness precisely, even at the sub-nanometer scale, by fine-tuning the LbL deposition parameters.

[0027] The PEM film deposition process as shown in Fig 1(a) involves inflow solutions of polycations (PAH) and polyanions (PAA) in an alternating mannerRSA23P0130(IDF-2732) into a microfluidic channel housing the EFBG interrupted by a DI water rinse. PAH solution is first injected into the channel through the respective tubing and the EFBG is kept dipped in the solution by closing the outlet knob. Following this, the solution is cleared out by opening the outlet and injecting DI water into the channel. The same procedure is then used for the deposition of PAA followed by rinsing. This buildup process is repeated alternatingly to deposit PEM complex over the surface of the EFBG and picturized in Fig 1(a) (inset). The deposition process is monitored in situ using the optical interrogator system, which measures the Bragg wavelength shift at a sampling rate of 1Hz. The increase in Bragg wavelength during the pumping of PAH and PAA solutions indicates the deposition of PEM, while a decrease in Bragg wavelength during the rinsing process is indicative of the desorption of PEM. This process is repeated to obtain the desired number of bilayers while continuously monitoring the Bragg wavelength. The notation EFBG -(PAH pH / PAA pH) n is used to represent the layer buildup of PEMs on EFBGs, where ‘n’ stands for the number of bi-layers being adsorbed on EFBGs and pH corresponds to the pH of the polyelectrolyte solution.

[0028] Results and Discussions

[0029] Modelling EFBGRSA23P0130(IDF-2732)

[0030] The EFBG was modeled using the electromagnetic waves frequency domain interface in the Wave Optics module in COMSOL Multiphysics. The Schematic of the EFBG design is shown in Fig 1(a) inset. The electromagnetic waves frequency domain interface uses the Finite element method (FEM) to solve the frequency domain form of Maxwell’s equations and provides the electric field distribution for the design structure. The effective refractive index (neff) obtained from the simulations is used to obtain the Bragg wavelength by using Equation 1. The simulation parameters utilized for the simulations are provided in Table 1. λB = 2neffΛ (1)

[0031] As the adsorbed polyelectrolyte thickness changes, the corresponding Bragg wavelength of the EFBG shifts. The Bragg wavelength for different values of adsorbed thickness was obtained, and an exponential growth model given in Equation 2 was used to obtain a relation between the two parameters. t= a(ebΔλB − 1) (2)

[0032] Where, t is the adsorbed thickness in nm, and a and b are the curve fitting constants. The values of the constants were obtained using curve fitting as a= 337.1 and b=0.773. The fit model is shown in Fig 1(b). Using this relation, the real-time changeRSA23P0130(IDF-2732) in Bragg wavelength data can be mapped to the thickness of polymer adsorbed over the EFBG. Table 1: COMSOL Multiphysics simulation parameters. Parameter Value Core diameter (a)4.2 ^^mCladding thickness (b) 0.55 ^^m Polyelectrolyte adsorbed thickness (c) 0 to 400 nm Polyelectrolyte medium diameter (d)12.6 ^^mRI of Core (^^^^^^^^^^)1.4749RI of Cladding (^^^^^^^^^^) 1.444 RI of adsorbed polyelectrolyte layer1.52

[0031] RI of Polyelectrolyte medium (^^^^^^^^^^^^^^)1.33

[0031] Pitch of the grating 532 nm Simulation wavelength 1550 nm

[0033] Table 1 gives the simulation parameters used during the simulation of the EFBG model in Electromagnetic Waves Frequency Domain (EWFD) interface of the Wave optics module in COMSOL Multiphysics. The polyelectrolyte adsorbed thickness is varied from 0-400 nm and effective refractive index (neff) is obtained.RSA23P0130(IDF-2732) Mechanism of layer growth:

[0034] PEM buildup at the surface of EFBG takes place largely due to consecutive alternative deposition of oppositely charged polymers causing a surface charge reversal at each deposition step, resulting in a gradual layer build-up of polyelectrolytes. The LbL assembly of PAH / PAA is due to the formation of polyelectrolyte complexes of polycations and polyanions via the ionic attractions between the carboxylate (COO-) and ammonium (NH3+) groups as shown in Fig 2(a). Adsorption of the PEM complex on the EFBG surface causes a change in neffof the EFBG. This causes a shift in Bragg wavelength as per Eq 1 which is monitored by using the measurement setup as shown in Fig 1(a). Fig 3 shows the shift in Bragg wavelength, monitored in-situ, during layer-by- layer adsorption of weak polyelectrolytes. Increase in the Bragg wavelength shows layer buildup of individual experiments of EFBG-(PAH5.5 / PAA5.5)n as well as EFBG-(PAH7 / PAA7)nself-assembly at pH 5.5 and pH 7. The Bragg wavelength shift data is converted to the adsorbed polyelectrolyte thickness using the FEM model in Fig 1(b) to obtain Fig 2(d). Fig 4 shows the deposition of individual polyelectrolyte layers, and Fig 5(a) and 5(b) define the conventions used for the various parameters used for the quantitative analysis in this paper.RSA23P0130(IDF-2732)

[0035] Fig 6(a) and 6(b) compare relative pre and post-rinse growth curves of (PAH5.5 / PAA5.5)n with (PAH7 / PAA7)nsystem respectively. Fig 6(c) and 6(d) compare the relative pre and post-rinse growth curves of the two systems with respect to the extracted thickness values from COMSOL simulations. The growth curves have been plotted for a set of 3 experiments for pH 5.5 and pH 7 systems (Error bars indicate the deviation in measurements). It was evident from both pre and post-relative growth curves that the total growth of pH 5.5 system was about 42% (43%) and 48% (50%) relatively higher than that of pH 7 system respectively. Another interesting observation was that, for both pH 5.5 as well as pH 7.0 systems there was an exponential growth. Error bars indicate one standard deviation from the mean value, n=3.

[0036] The buildup mechanism which leads to exponential growth can be explained by 'in and out’ diffusion of one among the two oppositely charged polyelectrolytes during the layer buildup as shown in Fig 2(c). This hypothesis was experimentally validated earlier, in which one of the polyelectrolytes, poly(L-lysine), diffuses into precursor layers during the adsorption step. poly(L-lysine) diffuses out during subsequent rinsing and continues to diffuse out even when dipped in oppositely charged HA, during which inter-polyelectrolyte complex is formed on the surface, resulting in the exponential growth ofRSA23P0130(IDF-2732) PEMs. The influence of low molecular weight PAA was also studied, and it was observed that PAA is able to diffuse “in and out” of the film during assembly, resulting in an exponential layer build-up. It was also observed for strong polyelectrolyte systems that the diffusion coefficient of polyanions decreases with their molecular weight in poly- (diallyldimethylammonium) / polystyrene sulfonate (PDADMA / PSS) system which causes a decrease the mobility of the diffusing species. The same theory is likely to apply to PAA / PAH system, where the low molecular weight mobile PAH polycations are able to diffuse throughout the film. The PEM thickness deposited is thus proportional to the number of mobile ions which have diffused in the film during the previous deposition step. The new thickness of the PEM film can be depicted as: δT(n+1) = k*T(n) (3)

[0037] where, δT(n+1) is the change in thickness on the deposition of a new layer, T(n) is the current thickness of the assembly, and k is a proportionality constant. (Picart, C. et al. Molecular basis for the explanation of the exponential growth of polyelectrolyte multilayers. Proc. Natl. Acad. Sci. 99, 12531–12535 (2002)). This results in the exponential growth of the PEM system. Hence, it can be concluded from the investigations discussed so far that the 'in and out diffusion' of PAH (low molecular weight) duringRSA23P0130(IDF-2732) the layer buildup process might be the possible reason for the exponential growth of (PAH / PAA) multilayer system. Additionally, the absolute desorption analysis for both pH systems was also performed.

[0038] Fig 7(a) and 7(b) show the absolute values of − an inter-sample mean of the desorbed mass (Δλdes) which is the difference between λpostand corresponding λpre.Fig 7(c) and 7(d) show the desorbed values of thickness (Δtdes) after each rinse for PAA and PAH. It is evident from Fig 5 that the decrement in the mass of PAH is exponential for both pH 5.5 and pH 7 systems, and the decrement in mass is linear for PAA in both systems. While the contribution of low molecular PAH chains to adsorption is more than that of PAA, their short chain lengths result in weaker attachment to the EFBG surface which is evident in its exponential desorption. All these observations emphasize that, irrespective of the assembly pH, 'in and out' diffusion of PAH may eventually lead to the exponential growth of PEMs. The in-out diffusion of PAA might not be observed as it has a higher molecular weight, and its mobility is suppressed due to its more coiled chain structure than its counter ions. pH, which plays crucial role in the degree of ionization of weak polyelectrolytes does not seem to cause any significant change in the desorption characteristics.RSA23P0130(IDF-2732)

[0039] Determination of mass during layer adsorption and desorption:

[0040] From desorption analysis shown in Fig 7(c) and 7(d), if the decrement in the adsorbed amount of PAH in both the systems was considered, the growth constant, b, of pH 5.5 is approximately 0.2309, whereas for pH 7 system, b is approximately 0.1672. This implies that the amount of PAH diffusing out from the PEMs is lesser in pH 7 system compared to that of pH 5.5 system. Fig 7 also shows the desorption curves for PAA of both the systems. The desorption of PAA is linear in both cases, but the slopes are different (-1.099 for pH 5.5 and -0.8511 for pH 7). This implies that relatively less amount of PAA is being used for layer buildup in pH 7 system than in pH 5.5 system. In order to understand the contribution of each polyelectrolyte in the layer buildup mechanism the mean-based mass adsorption and desorption calculations were performed for both pH systems. Γ MFs=actadΓinit(4) MFdes

[0041] Using Equation 4 & 5, Mass fraction analysis was performed for both adsorbed (MFads) and desorbed (MFdes), The initial mass (Γinit) is the mass of polyelectrolytes that adsorbs to the surface before rinsing. The actual adsorbed mass (Γact) is the mass retained on the surface as molecularly thin filmRSA23P0130(IDF-2732) after successive post rinse process. Fig 8(a-b) shows the plot for mass fraction analysis for both the pH systems. It is evident from Figure 8(a) that approximately 60% of mass fraction of PAH and PAA participates in layer buildup of pH 5.5 system and only around 20 to 30% mass fraction of polyelectrolytes are participating in layer buildup of pH 7 system (Fig 8(b)). As the exponential growth depends on the mass of PAH that possibly diffuse 'in and out' of polymer stack and hence the mass fraction of adsorption might be higher in pH 5.5 system than in pH 7 system.

[0042] Kinetic analysis: Analysis of the kinetics of LbL assembly of pH 5.5 and pH 7 systems was performed by considering the meantime taken by each polyelectrolyte to reach 90% of their saturation thickness during each layer buildup. Fig 9(a) shows the quantified time analysis of pH 5.5 and pH 7 systems. For pH 5.5 system, the adsorption time of PAH for 6thlayer is around five minutes while, PAA requires thirteen minutes to reach the 90% of its saturated thickness. Even for the pH 7 system, the adsorption time for 6thlayer of PAH is less than PAA by around a minute. These findings illustrate that PAH adsorption is faster than PAA due to its lower molecular weight and associated electrostatic interactions. On the contrary, high molecular weight PAA suppresses the interlayer diffusion while slowing down the deposition significantly due to the highly coiledRSA23P0130(IDF-2732) structure of high molecular weight PAA chains. The difference in the saturation time for different pH, is due to change in the degree of ionization, which contrasts the adsorption due to the electrostatic interaction between the multilayered surface and the incoming polymeric chain.

[0043] The results of this experiment point to the many advantages of EFBG, including its low noise, high sensitivity, and real-time measurements. Fig 9(b) shows a comparison of the popular PEM thickness monitoring techniques with respect to their limit of detection. Ellipsometry is one of the most popular methods for thickness characterization and can be used for sub-nanometer thickness measurements, but requires the optical properties of the bulk film to be known in advance, and the substrate roughness results in uncertainty in the measurement, especially for ultrathin films below 25 nm. QCM and AFM are the popular non-optical techniques for monitoring PEM buildup. Although AFM gives absolute thickness measurement, it involves measurement based on the height difference between the substrate and the film's surface by creating a step patterning or a ‘tip scratch’. QCM, on the other hand, can provide inaccurate thickness measurement due to changes in mass on account of adhered liquids.

[0044] EFBG not only allows us to observe PEM buildup during the initial layers at sub-nanometer (10 nm)RSA23P0130(IDF-2732) scale but also provides compelling evidence for the in and out diffusion of lower molecular weight species during the adsorption of PEM complex at the dipping and desorption phases. Confocal microscopy, which has been widely used for reporting in and out diffusion, has limited sensitivity due to the diffraction limit, and PEM films need to be deposited to several micrometers thickness while FRAP and FRET require fluorescent tagging of diffusing species. Non-optical methods like XPS and SIMS can be used to study inter layer diffusion, but they are destructive techniques that lead to change in the nativity of the of deposited PEM films. EFBG can be used as a standalone optical technique for thickness measurement and probing the in and out diffusion thereby helping us to analyze the influence of various factors that determine PEM film buildup, thus enabling and expediting their utility in various applications.

[0045] Methods Materials: The polyelectrolytes, PAH of Mw~15 KDa, and PAA 150 KDa were acquired from Sigma-Aldrich. The Sodium Chloride (NaCl), Hydrochloric acid (HCl), and Sodium Hydroxide (NaOH) were acquired from Merck for adjusting ionic concentration and pH. Polyelectrolyte solutions were prepared for ionic strength of 0.1M and 0.01M concentration based on repeat unit molecular weight using 18.2MΩ ultra-pure de-ionized (DI) water from a MilliporeRSA23P0130(IDF-2732) purification system. The optical fiber SM1500 was procured from Fibercore Inc., USA.

[0046] PEM in-situ monitoring using Optical EFBG technique: The fiber Bragg gratings were inscribed in the core of fiber SM1500 over a length of about 3 mm using a custom grating inscription setup. A UV laser (KrF excimer laser) at 248 nm wavelength passes through the phase mask to form an interference pattern in the core of a photo sensitive optical fiber. This results in the photo- imprinting of a refractive index modulation (Bragg grating) in the fiber core. To increase the interaction of the propagating optical field in the fiber core with the surrounding medium, the cladding region is etched using hydrofluoric (HF) acid at 40% concentration.

[0047] In-situ studies were performed by alternatively dipping the EFBGs in cationic polyelectrolyte solution (PAH) and anionic polyelectrolyte solution (PAA), with intermittent rinsing steps. The dipping time was kept at 20 min, followed by a 5 min rinsing cycle with DI to remove any loosely bound polyelectrolyte ions.

[0048] Optical Sensing Interrogator System: SM130 from Micron Optics Inc., USA was used for Optical interrogation, which consists of a 10mW light source centered around 1550 nm with a 40 nm bandwidth and a high-resolution opticalRSA23P0130(IDF-2732) spectrum analyzer. The resolution of this system is 1 pm at a sampling rate of 1 kHz.

[0049] Characterization Scanning Electron Microscopy: The surface morphology studies were performed using field emission scanning electron microscope (FESEM) from Zeiss. For SEM studies, the PEMs were desiccated overnight and the dried samples were sputtered with 2nm of gold using the sputtering unit from Zeiss. The images were taken using an ’in-lens’ detector of SEM at a working distance (WD) of 1:5mm ≤ W D ≤ 5mm. The beam accelerating voltage was kept ≤ 15KeV.

[0050] Ellipsometry: The thickness of PEMs was measured using M-2000, a variable angle spectroscopic ellipsometer from J.A. Woollam. Accurate measurements of the thickness of polymers need careful modelling techniques to obtain accurate results. Unknown optical parameters, thickness, and complex refractive index, of the PEM film are obtained by fitting the experimental Ψ and ∆ values to Fresnel equations using extensive modelling techniques.

[0051] In-situ studies on layer buildup mechanism were performed on weak polyelectrolyte-based PEMs with EFBGs. The studies on weak polyelectrolyte systems points to an 'inter diffusive' behaviour of low molecular weight PAH leading to the exponential growth of PEMs. The studies on adsorbed and desorbedRSA23P0130(IDF-2732) mass, at different assembly pH, emphasize that the modulation in optical properties like, thickness of PEMs, can be attributed to the differences in the extent of 'in and out' diffusion of PAH in the PEMs at different assembly pH conditions. EFBG measurement is dependent on the Bragg wavelength shift which can be measured accurately up to 1 pm accuracy. This allows us to monitor the PEM deposition at nano-scale giving significant insight into the in and out diffusion phenomenon at the initial layer buildup. The measurement setup is also relatively inexpensive and can be applied for analysis of films with a wide range of thicknesses. The EFBGs in-situ method is extremely versatile in studying kinetics of the sequential deposition of self-assembled species, and can help in tuning the parameters in multilayer build-up process and to explore in bio-molecule adsorption and desorption process.

Claims

RSA23P0130(IDF-2732) We Claim:

1. A device (100) for measuring sub-nanometer thickness and monitor the inter-diffusive of any charge based material on any surface, the said device (100) comprising of a fluidic channel (1) on the surface (2); an optical fibre (3) positioned on the fluidic channel (1); means (4) to deposit one or more charged layers (5) of one or more material on the optical fibre (3); a light source (6) to pass through the optic fibre (3); and an optical interrogator system (7), characterized in that, the cladding (8) of optical fiber (3) acts like Etched Fiber Bragg Grating (EFBG) sensors, and the thickness of the deposition layer (5) is detected by the optical interrogator system (7) by detecting the shift in Bragg wavelength.

2. The device as claimed in claim 1, wherein the said measured thickness is used to monitor individual layer growth and control the film thickness precisely.

3. The device as claimed in claims 1 and 2, for use as an instrument for probing inter-diffusive behaviour of macromolecules, bio-polymers and proteins at sub-nano scale.

4. A method of measuring sub-nanometer thickness and monitor the deposition of any material on any surface, the said method comprising the steps of: providing a fluidic channel (1) on a surface (2); positioning an optical fibre (3) in the fluidic channel (1), wherein the optical fibre (3) is etchedRSA23P0130(IDF-2732) with grating to act like Etched Fiber Bragg Grating (EFBG); depositing one or more layers (5) of the material on the optical fibre; passing light through the Etched Fiber Bragg Grating (EFBG); and measuring the shift in the Bragg wavelength of the light to determine the thickness of each deposition (5).

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