Introduction:
Forensic science is a term which describes the application of sciences for the crime scene inspection and gathering of evidence to be used for investigation of crimes such as murder, theft, fraud or terrorism activities. It is multidisciplinary field and its major purpose is to assure law enforcement in society. It is also used to analyze the possibility of the presence of chemical warfare agents or high explosives, oversee conformity with international agreements regarding weapons of mass destruction, or to test for propellant stabilizers. Forensic science encompasses mainly following areas of science; biology, chemistry, and medicine, it also includes the use of physics, computer science or psychology. At crime scene, the objects, substances (including blood or drug samples), chemicals (paints, explosives, fire accelerants, toxins), traces (hair, fibres, skin), or impressions (fingerprints, tool marks or tidemarks) are collected as evidence.
A growing area of forensic science is the analysis and early detection of possible terrorist attacks, or breaches of security. There is a wide range of samples taken from the scene of suspected chemical or biological weapons to be analyzed, but the method of analysis slightly different from a criminal investigation. These samples often contain very minute amount of chemicals and require very accurate and sensitive analytical instruments. In addition to the already-described samples, evidences of weapons of mass destruction are obtained by collecting swabs from objects, water, and plant material. After that they are tested for the detection of radioactive isotopes, toxins, or poisons, as well as chemicals that can be used in production of chemical weapons. Forensic chemistry performs qualitative and quantitative analysis of chemicals found on people, various objects, or in solutions. The chemical analysis is the most varied from all the forensic disciplines. Chemists analyze drugs as well as paints, remnants of explosives, fire debris, gunshot residues, fibers, and soil samples. They can also test for a presence of radioactive substances (nuclear weapons), toxic chemicals (chemical weapons), and biological toxins (biological weapons) [1, 2].
Nanotechnology is the understanding and control of matter generally in the 1-100 nm dimension range. It is a multidisciplinary field, which covers an enormous and diverse range of devices derived from biology, engineering, chemistry, physics and even forensic science. The application of nanotechnology to forensic science is also referred as nano-forensics; it concerns the use of very precisely designed materials at nanoscale to develop novel techniques of collection and assay of forensic evidences. Nanomaterials have novel physiochemical properties, like large surface area to mass ratio, ultra small size, and high reactivity. These properties make them different from bulk materials having same composition and can be used to surmount the restrictions found in conventional methods for assay of forensic evidences. Nanotechnology in forensics promises new approaches for earlier detection, collection and analysis of forensic evidences.
Many techniques have been developed and applied for the synthesis of nanoparticles but majorly, there are mainly two approaches toward the synthesis of nanosized materials; top-down and bottom-up approaches.
Top-down approach includes milling or attrition, repeated quenching, and lithography. In this approach bulk materials are modified to give small features and such prepared nanoparticles are commonly used in the fabrication of nanocomposites and nanograined bulk materials.
Bottom-up approach includes plasma, laser, liquid phase, flame spray synthesis. In this approach, these "self-assembly" synthesis methods usually ensue in well controlled nanoparticles, when small building blocks are congregate into larger structure, allowing fabrication of nanoparticles or the formation of composite materials with a very narrow size distribution.
Bottom-up approach is far more popular because nanoparticles can be synthesized by confining chemical reactions, nucleation and growth processes in a small space such as micelles. But for any practical application, the processing controlled conditions are needed so that the resulting nanoparticles have the following properties: (1) particles should be monosized or with uniform size distribution, (2) identical shape or morphology, (3) identical chemical composition and crystal structure, so that core and surface composition must be the same, and (4) individually dispersed nanoparticles. For the synthesis of nanoparticles, various methods or techniques can be grouped into two categories: kinetic approach and thermodynamic equilibrium approach.
In the thermodynamic approach, synthesis process consists of (1) generation of super-saturation, (2) nucleation, and (3) subsequent growth of nanoparticles. In the kinetic approach, nanoparticles are synthesized by either confining the process in a limited space such as aerosol synthesis or micelle synthesis, or limiting the amount of precursors available for the growth such as used in molecular beam epitaxy [3].
Synthetic Techniques:
Nanoparticles can be synthesized by various methods; some of these techniques are discussed as following:
Hydrothermal Synthesis
Sol-Gel Process
Sonochemical Processing
Co-Precipitation Synthesis
Homogenous Precipitation Method
Micro Emulsion Method
Hydrothermal Synthesis:
It is synthesis method which depends on the solubility of material in water at elevated temperature and pressure. At high temperature, water transforms the precursor material into nanoscale product. This process occurs in an apparatus consisting of autoclave, temperature and pressure gradient is maintained in it so that it is more than equilibrium water vapor pressure. This process make possible the easy and precise control of the crystallinity, shape and distribution size, of the final product by adjusting the reaction parameters such as temperature, reaction time, type of solvent, precursor and surfactant [10].
Sonochemical Processing:
Sonochemical process has been proved a useful method for nanoparticle synthesis. Ultrasounds have frequency in the range 15 kHz-1 GHz. In this method, ultrasonic waves of 1-10,000 µm wavelength are generated in the aqueous reaction. Acoustic waves have not any molecular dimensions to couple with reaction molecules. Therefore very small cavities are generated and bubbles are formed in reaction. After reaching the critical size, the bubble collapses adiabatically forming a hot spot of very high temperature (>5000°C) and pressure (>2000 atm). Nanoparticles of same size as hot spots are then formed in these spots.
Surfactant:
The word surfactant is the combination of three words surface-active-agent. When they are added to any (non-aqueous or aqueous) liquid, alter or modify the interfacial and surface properties of the liquids. These peculiar characteristics of surfactants are due to their amphiphilic character. Each surfactant molecule is consisted of two parts; hydrophilic head and a hydrophobic (or lipophilic) tail.
Hydrophilic head
Hydrophobic tail
Fig. 1 Surfactant molecule
Classification of Surfactants:
Surfactants are classified on the basis of charge on hydrophilic head. They are generally divided into four main classes:
Anionic Surfactants
Cationic Surfactants
Nonionic Surfactants
Ampholytic or Zwitterionic Surfactants
Anionic Surfactant:
These are the most common and ancient surfactants and produced in large quantity due to their low cost production and account for 50% of production in world. They have excellent cleansing power for the removal of dust particulates. In water, they are dissociated into in an amphiphilic cation and anion, generally quaternary ammonium ions or alkaline metal (Na+, K+). They include alkylbenzene sulfonates, Fatty Acid Isethionates and Taurides, soaps and detergents (lauryl sulfate), wetting agent (sulfonic acid derivatives) etc.
Cationic Surfactants:
Such type of surfactants usually have halogen group, nitrogen containing compounds like ethoxylated alkyl amines and quaternary ammoniums, long chains of alkyl amines and alkyl imidazole.
Nonionic Surfactants:
Nonionic surfactants do not ionize in aqueous solutions. They have hydrophilic groups like alcohols, esters, glycerols, amine oxides and alkyl amines.
Ampholytic or Zwitterionic Surfactants:
These type of molecules exhibits both types of characteristics in aqoueas solution; anionin and cationic. They have hydrophilic head groups consisting of imidodiacids acyl ethylenediamines and their Derivatives.
Table: Classification of Surfactant depending upon Hydrophilic Group
Surfactant Types
Examples
Anionic Surfactants
Acylamino Acids and Salts
Acyl Glutamates
Sodium acylglutamate
Acyl Peptides
Sodium acyl polypeptide
Acyl Sarcosides
Acylamino acid
Carboxylic Acids and Salts
Carboxylic Acids
Magnesium alkyl carboxylate
Ester carboxylic Acids
Sodium dilaureth-7 citrate
Ether Carboxylic Acids
Alkyl polyglycol ether carboxylate, sodium salt
Sulfuric Acid Derivatives
Alkyl Sulfates
R-OS03H-H2NCH2CH20H
MEA - alkylsulfate
Alkyl Ether Sulfates
RO-(CH2-CH2-O)n~S0-3 -Na+
Sodium alkyl ether sulfate
Sulfonic Acids and Salts
Alkyl Sulfonates
Secondary sodium alkyl sulfonate
Alkyl Aryl Sulfonates
Linear alkylbenzene (LAB)
Sulfosuccinates
Sodium alkyl sulfosuccinamate
Sulfo Fatty Acid Esters
Methyl ester of a-sulfofatty acid, sodium salt
Fatty Acid Isethionates and Taurides
Tauride
Phosphoric Acid Esters and Salts
Sodium ethoxylated alkyl phosphate
Cationic Surfactants
Alkyl Amines
Alkylamido dimethyl propylamine
Alkylimidazolines
R' = CH2CH2NH2 => alkyl aminoethyl imidazoline
Quaternary Ammonium Compounds
Tetraalkyl(-aryl) Ammonium Salts
Tetraalkyl(-aryl) Ammonium Salt
Heterocyclic Ammonium Salts
Imidazolinium quaternary compound
Alkyl Betaines
Alkylamidopropyl betaine
Ethoxylated Alkyl Amines
Alkyl propanediamine ethoxylate
Esterified Quaternaries
Esterquat
Nonionic Surfactants
Alcohols
Ethers
Alkoxylated Alcohols
alcohol ethoxylate
EO/PO Block Polymers
Ethoxylated PPG ether
Alkylpolyglucosides
Alkylpolyglucoside
Ethoxylated Oils and Fats
Ethoxylated products of lanolin and castor oil
Alkanolamides
Alkanolamides
Monoalkanolamide
Ethoxylated Alkanolamides
Polyethoxylated monoalkanolamide
Esters
The ethoxylated fatty acids
PEG fatty acid ester
The esters, glycerol, glycol esters and ethoxylated derivatives
Polyethoxylated l,3-diglyceride
The sorbitan esters and ethoxylated derivatives
1, 4 -Sorbitan monoester
The alkyl carbohydrate esters
Saccharose fatty acid monoester
Amine Oxides
Amine Oxide
Ampholytic Surfactants
Acyl Ethylenediamines and Derivatives
Acylamphoacetate
N-Alkyl Amino Acids or Imino Diacids
Aminopropyl alkylglutamide
Micelle Formation and Critical Micelle Concentration:
When surfactant molecules in aqueous solution attain a minimum free energy state and form micelle, that point is called critical micelle concentration (CMC), During micelle formation the hydrophilic head and a hydrophobic and lipophilic tail of surfactant molecule arrange themselves to form micelle. On the basis of nature of solvent, there are two major types of micelle; Normal Phase Micelle and Inverse Phase Micelle.
Normal Phase micelle is formed in polar aqueous solution i.e. water etc. In this type, the hydrophilic heads of surfactant molecules assemble with solvent molecules in such a way that, lipophillic tail sequester inward.
The inverse phase micelle formation is completely reverse of normal phase. The tail interacts with solvent molecules and lipophobic head sequester inward.
Characterization Techniques:
Different techniques have been used for properties and size determination of nanoparticles. The methods using SEM, TEM, X-ray diffraction techniques and conductivity measurements provide information on particles, for example, X-ray diffraction can determine internal structure and size of particles.
Electron Microscopy
Electron Microscopy is the major technique used for nanoparticle size determination.
There are two main types of electron microscopy:
SEM (Scanning Electron Microscopy)
TEM (Transmission Electron Microscopy)
Scanning electron microscopy
This technique is majorly used for the study of particle surface. Magnetic lenses give a thin probe (1-10 mm) by constricting an electron beam, which travels over sample point-by-point progressively and scan it. Several types of emission are produced by the interaction of electrons with the surface i.e. Secondary and reflected electrons, transmitted electrons, X-ray slowing down radiation and optic radiations. These types of radiation can be converted into electrical signals, which are then amplified and taken to a cathode-ray tube. Finally, the Images formed on screen are photographed. This technique provides the great body of information but it took long scanning time to give high resolution.
Transmission electron microscopy
In vacuum conditions of ca. 10-6 mmHg, a strong beam of accelerated electrons is passed through thin film of sample of with an energy of 50-200keV. A very thin film of about 0.01-µm thickness is prepared so that the electrons can penetrate through it. Some electrons are passed through the sample and others are deflected at very small angles by the atoms in sample. These electrons get into a system of magnetic lenses and form a bright-field image of internal structure of sample on a screen. The resolution of 0.1 nm is attained, which is in agreement to a 106 magnification factor. It depends on the method of preparation of the sample and its resolution nature. TEM has made feasible to acquire diffraction patterns, which give useful information of the crystalline structure.
Energy Dispersive X-Ray Analysis:
Energy Dispersive X-Ray Analysis, often referred as EDX, is the technique used in combination with scanning electron microscopy (SEM). It gives information about near surface elemental composition of a sample at different positions. When electron beam of energy 10-20 keV, strikes with the sample surface, x-rays of different intensities are emitted. The electron beam moves across the sample surface and emitted x-rays form an image. The image can take hours to form if the intensity of x-rays is very low. The EDX analysis produces spectral data showing peaks of the elements present in the sample being analysed [9].
Atomic force microscopy (AFM):
Most atomic force microscopy (AFM) can measure roughness and texture of surface of materials at nanoscale. A laser beam strikes to the cantilever surface with an angle and reflects into the position sensitive photodiode. A probe consisting of a sharp tip is located at the end of cantilever; it scans across the sample using piezoelectric scanners. This causes cantilever to bend and a variance between signals is prompted from photodiode. When tip is in contact with sample the cantilever is pushed against the surface of sample, this deflection of the lever is measured and images are formed.
X-Ray Diffraction:
X-rays are high energy electromagnetic radiations; disperse when come in contact with metal particles, crystals or other molecules into various directions. The intensity of these refracted rays relies on the shape and size of particles of sample. This technique allows the determination of elemental composition, size, shape, texture, symmetry and even atomic structure of sample. This all information is presented by complex analysis and complicated mathematical processing of reflections of all diffracted intensities.
Hazardous Chemicals:
Fig. 1 Classification of Explosive based on structure and explosion rate
Explosives:
Explosives are such unstable chemical compounds (chemical or nuclear) which can be initiated to undergo very rapid and decomposition resulting in the high release of heat or the development of sudden pressure effect and formation of more stable material. Explosives have been classified into many types on the basis of structure and performance (Fig. 1). Explosives are classified as low and high explosives on the basis of their detonation velocities (burn rates) and these types are further classified into different forms.
Low explosives that detonate at low rates (cm s−1) include propellants, pyrotechnics, smokeless powder, black powder, etc. High explosives detonate at very high velocities of km s−1, the chemical reaction propagates with such rapidity that it exceeds the velocity of sound. High explosives have again been sub-divided into two groups, i.e. primary explosives and secondary explosives. Primary explosives, often referred as 'initiating explosives' are highly shock sensitive and can be used to ignite secondary explosives i.e. lead azide and lead styphnate. Secondary explosives, which include nitroaromatics and nitramines are used as main charge or bolstering explosives much more prevalent at military sites than primary explosives. They can be further categorized into melt-pour explosives and plastic bonded explosives. Melt-pour explosives are based on nitroaromatics, such as trinitrotoluene (TNT), dinitrotoluene (DNT) and plastic bonded explosives are based on a binder and crystalline explosive formulated with one or more high explosives, such as hexahydro-1,3,5 trinitroazine (RDX).
Classification on the basis of chemical nature:
The propellants and explosives are mostly organic compounds and can be classified into following based on their chemistry: (1) Nitramines or nitrosamines, such as octogen (HMX) or RDX; (2) Azide explosives (3) Organic peroxides, such as HMTD [hexamethylenetriperoxidediamine], also known as home-made explosives (HMEs) (4) Nitroaromatic compounds, such as TNT, dinitrobenzene (DNB), hexanitrostilbene, picric acid (5) Nitrate esters, such as pentrite (PETN), ethylene glycol dinitrate (EDGN), nitroglycerine, and nitroguanidine (NQ) [4].
The energetic material used by the military as propellant and explosive are mostly organic compounds containing nitro (-NO2) groups. Identification, quantification and remediation of explosives have become a highly significant task in forensic science, anti terrorist activities and global demining projects. There are two major threats from these nitroexplosives. One of the threats is their illegal use for terrorism, which will cause chaos in the nation, other one is the health associated risks with the release of these compounds in environment. The nitroaromatics has a special characteristics or ability to penetrate in the skin causing the formation of methemoglobin on acute exposure and severe anaemia on chronical exposure.
The problem of contamination of soil and groundwater by nitroaromatic compounds is a widespread environmental concern with environmental deterioration. These compounds have several applications in agricultural, industrial, and military and assessments of the hazards from these applications quite often do not take into account chemical processes [5].
