Fluorescence Microscopic Image of RhB18-NK in Lanmguir monolayer

NK-RhB18 in Langmuir monolayer

Fluorescence Microscopic Image of RhB18-NK in Langmuir film

Organo-clay hybrid films

Recently we have prepared organo-clay hybrid films using Layer-by-Layer self assembled technique and Langmuir-Blodgett technique.

Hi

My recent paper at Elsevier Journal

Title: Preparation and characterization of an anionic dye–polycation molecular films by electrostatic layer-by-layer adsorption process

D. Dey1, S. A. Hussain1, R. K. Nath2 and D. Bhattacharjee1
1. Department of Physics, Tripura University, Suryamaninagar 799130, Tripura, India
2. Department of Chemistry, Tripura University, Suryamaninagar 799130, Tripura, India Journal: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Abstract:
This communication reports the formation and characterization of self-assembled films of a low molecular weight anionic dye amaranth and polycation poly(allylamine hydrochloride) (PAH) by electrostatic alternating layer-by-layer (LBL) adsorption. It was observed that there was almost no material loss occurred during adsorption process. The UV–vis absorption and fluorescence spectra of amaranth solution reveal that with the increase in amaranth concentration in solution, the aggregated species starts to dominate over the monomeric species. New aggregated band at 600 nm was observed in amaranth–PAH mixture solution absorption spectrum. A new broad low intense band at the longer wavelength region, in the amaranth–PAH mixture solution fluorescence spectrum was observed due to the closer association of amaranth molecule while tagged into the polymer backbone of PAH and consequent formation of aggregates. The broad band system in the 650–750 nm region in the fluorescence spectra of different layered LBL films changes in intensity distribution among various bands within itself, with changing layer number and at 10 bilayer LBL films the longer wavelength band at 710 nm becomes prominent. Existence of dimeric or higher order n-meric species in the LBL films was confirmed by excitation spectroscopic studies. Almost 45 min was required to complete the interaction between amaranth and PAH molecules in the one-bilayer LBL film.

Keywords: Adsorption; Deposition process; Multilayer; Layer-by-layer (LBL) self-assembled films; UV–vis absorption and fluorescence spectroscopy
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recent publications from our group

One of our paper has been published in the Journal of Luminescence (Elsevier Journal)

Title: Photophysical characterizations of 2-(4-biphenylyl)-5 phenyl-1,3,4-oxadiazole in restricted geometry
P.K. Paula, S.A. Hussaina and D. Bhattacharjee
Journal of Luminescence
Volume 128, Issue 1, January 2008, Pages 41-50

Abstract:
Langmuir and Langmuir–Blodgett (LB) films of non-amphiphilic 2-(4-Biphenylyl)-5-phenyl-1,3,4-oxadiazole (abbreviated as PBD) mixed with stearic acid (SA) as well as with the inert polymer matrix poly(methyl methacrylate) (PMMA) have been studied. Surface pressure versus area per molecule (π–A) isotherm studies suggest that PBD molecules very likely stand vertically on the air–water interface and this arrangement allows the PBD molecules to form stacks and remain sandwiched between SA/PMMA molecules. At lower surface pressure, phase separation between PBD and matrix molecules occurs due to repulsive interaction. However, at higher surface pressure, PBD molecules form aggregates. The UV–vis absorption and steady-state fluorescence spectroscopic studies of the mixed LB films of PBD reveal the nature of the aggregates. H-type aggregate predominates in the mixed LB films, whereas I-type aggregate predominates in the PBD-PMMA spin-coated films. The degree of deformation produced in the electronic levels are largely affected by the film thickness and the surface pressure of lifting.
 

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Immobilization of single strand DNA on solid support

Recently we have demonstrated the immobilization of single strand DNA onto a solid support by electrostatic interaction with a polycation poly(allylamine hydrochloride) (PAH). The films were analyzed by UV–vis spectroscopy. We have shown that single stranded DNA gets immobilized on the PAH backbone of Layer-by-Layer (LbL) films when the films are fabricated above the melting temperature of DNA. Singly stranded DNA immobilized in the LbL films is not restored into double stranded DNA at room temperature.

Read the article here at Chemical Physics Letters

SEM image of 10 layer RB-PAH LbL films

AFM image of hectorite assembled onto quartz slide by Langmuir-Blodgett Technique

 

 

Can we see the atom or something of nano dimension?

            Twenty five years ago, this question was often met with amazement by common people and with scepticism by the teaching community. The answer may be ‘atom is something which is not really visible’. But nowadays, it’s a common phenomenon to visualize the atom, even seeing them moving. Its not really a challenge to see the atom but to know and visualize how they are arranged within matter. This is because it’s the arrangement of that determines the properties of materials- their hardness, ductility, brittleness, strength and the catalytic properties.

             With our eyes we can see a fraction of a millimeter, let’s say one tenth of a millimeter or one hundred micrometers which is a little more than the diameter of a single hair. If we want to see at smaller scales than this, we use a lens. The lens bends the light rays or refracts and the object appears larger. Optical microscopes work on this principle. Lenses work well as long as we do not have to work at scales smaller than the wavelength of light which is about half a micrometer. This is because the smallest object that can be seen with waves of wavelength  is about  in dimension (the Rayleigh criterion). At this scale, we have to take into account the wave nature of light, so things are different. Optical microscopes help us to image about hundred times smaller than what we can see with our naked eyes. If we want to track a molecule or a particle that is still smaller, we can improve the resolution by labeling it with a fluorescent molecule. The fluorescence can be detected and we can follow the movement of the molecule or particles in liquids for example. If we need higher resolution, we can use electrons instead. Modern electron microscopes can let us see the arrangement of atom within the matter.  Thinking of electrons one might be puzzled as to how electrons, which are particles, can be used to image things at very small scales. At small scales things are different, drastically different. It’s the realm of quantum physics. Quantum particles propagate like waves and interact like particles. When the sample is thin enough and the electron comes in at high speed it can go through the sample. The path of the electron can be influenced by magnetic and electric fields and this effect is used in producing lenses for electrons. The advantage of electrons is that their wavelength is several orders of magnitude smaller than optical wavelengths and the microscope resolution is not limited by the wavelength but by the performance of the lenses. The wavelength of an electron is related to its velocity by  (de Broglie’s equation non-relativistic),  is the Planck’s constant,  and  are the mass and the velocity of the electron respectively. Now if  is greater than 80% of the speed of light, then  is less than  of the atomic dimension. With electron microscopes we can increase the resolution by more than a factor of thousand when compared to optical microscopes. We can see one tenth of a nanometer with a high resolution electron microscope, the length scale of the atom world. We can see planes of atoms. Electrons are also reflected off and in fact there is a type of electron microscope that works in reflection mode (SEM). SEM’s have about ten fold smaller resolution than TEMs. The drawback of electron microscopes is that electrons are charged and we need vacuum environment in order for the electrons to propagate freely. There are SEMs available today that do not need high vacuum and can function in a partial nitrogen or argon atmosphere with reduced resolution. This makes it particularly interesting to observe growth processes at the nanometer scale. Non conducting samples are more difficult to observe with electrons and often need to be coated with a conducting metal layer. The electron microscope also needs a high voltage generator (10kV to several 100kV) to accelerate the electrons. All together electron microscopes are rather large instruments.

For example, in silicon nitride ceramics the arrangement of lanthanum ( ) atoms (arrowed in figure 1) at the interface between a glassy film (top) and a silicon nitride crystal (bottom) is shown. The  atoms play a role in controlling the path of cracks as they pass through the silicon nitride ceramic.

     

Figure 1: The interface between a silicon nitride crystal (bottom) and a glass (top) in a silicon nitride ceramic. The bright atoms at the interface are La, used to engineer the strength of the interface.

            However it is the Atomic Force Microscope (AFM) (Nobel prize in 1986 for Binning and Rohrer for the fundamental Scanning Tunnelling Microscope) made revolutionary advancement towards the visualization of nanometer order and atomic dimension. The AFM works in the same way as our fingers which touch and probe the environment when we cannot see it. By using a finger to “visualize” an object, our brain is able to deduce its topography while touching it. The resolution we can get by this method is determined by the radius of the fingertip. To achieve atomic scale resolution, a sharp tip (radius ~1-2 nm) attached to a cantilever is used in the AFM to scan an object point by point and contouring it while a constant small force is applied to the tip. With the AFM the role of the brain is taken over by a computer, while scanning the tip is accomplished by a piezoelectric tube.

 Figure 2: The principle of Atomic Force Microscopy

 

The atomic force microscope got its name from the interactions between probe and sample on the atomic level The attractive Van-der-Waals-forces and repellent equal electric charges are described by the Lennard- Jones- potential. The AFM is based on a relatively simple mechanical principle: if you drag a very fine and flexible pointed needle (called a cantilever) over uneven surface, then the forces acting on the cantilever will be directly related to the displacements (deflections) of the cantilever. These deflections can be measured by shining an ultra sensitive laser on the tip of the cantilever and looking for changes in the reflected beam as the cantilever changes its position slightly. Under ideal conditions, this technique allows lateral resolution on the atomic level. The vertical resolution is below 0,1 nm.

Langmuir-Blodgett (LB) Films of water-soluble materials

 

Ideal LB compatible material is amphiphilic molecules having a hydrophilic (water loving) head group and a hydrophobic (water hating) tale part. Due to this amphiphatic balance these types of materials float onto water.

 

However recent observation reveal that good quality LB films can be prepared using water soluble materials. Recently we prepare the LB films of water soluble CTAB. First of all a model monolayer of a suitable amphiphilic molecule was prepared on the air-water interface. Then the solution of the sample molecule was spread from the back side of tha barrier. The water-soluble sample molecules come into the contact of the molecules of the model monolayer and interacted and forms complex. Thus a complex Langmuir monolayer of water soluble material is formed. Once this complex Langmuir monolayer is formed then it is easy to form mono- and multilayer LB films of this complex monolayer (Journal of Colloid and Interface Science 311(2) (2007) 361-367).