Introduction:

Definition: Sterilization is defined as complete removal of microorganisms from an object, surface or a product.
Chemical Sterilization Causes of removal of microorganisms by the use of chemical bactericidal agents.
Although the physical methods of sterilization are more appropriate it is not possible to use them in certain conditions e.g. in case of thermolabile materials, in such cases Chemical sterilization is preferred.
However compatibility between chemical agent and substance to be sterilized must be first taken into the consideration.
It is important to follow safety rules in the workplace safety during the use of chemical agents.
The chemical method of sterilization can be classified as,

Gaseous Sterilization:

Ethylene oxide (EO) gas is a common gas used for chemical sterilization because of its wide range of compatibility.
This method is a widespread method used for almost 70% of all sterilizations and around 50% for disposable medical devices.
Mechanism of antimicrobial action:

Causes alkylation of sulfhydryl, amino, hydroxyl, and carboxyl groups on proteins and imino groups of nucleic acids.

EO sterilization is usually conducted at the temperature range of 30-60°C for several hours which aids in the activity of the gas.
The efficacy of the gas depends on the concentration of gas available for each article which is greatly assisted by the good penetrating nature of the gas.
Ethylene oxide kills all known microorganisms, such as bacteria (including spores), viruses, and fungi (including yeasts and molds), and is compatible with almost all materials even when repeatedly applied.
Drawback:

Gas gets absorbed by the material, and the treated articles need to undergo a process of desorption to remove the toxic residual wastes.
Organisms are more resistant to ethylene oxide treatment in a dried state.

Formaldehyde is another important highly reactive gas which is used for sterilization.
This gas is obtained by heating formalin (37%w/v) to a temperature of 70-80°C.
It possesses broad-spectrum biocidal activity and is used in the sterilization of reusable surgical instruments, specific medical, diagnostic and electrical equipment, and the surface sterilization of powders.
Mechanism of antimicrobial action:

Causes alkylation of sulfhydryl, amino, hydroxyl, and carboxyl groups on proteins and imino groups of nucleic acids.

Has poor penetration power as compared to the ethylene oxide.
As a result of the low penetrating power, its use is often limited to paper and cotton fabrics.
Formaldehyde can generally be detected by smell at concentrations lower than those permitted in the atmosphere and thus can be detected during leakage or other such accidents.

Nitrogen dioxide is a rapid and effective sterilizing agent that can be used for the removal of common bacteria, fungi, and even spores.
NO2 has a low boiling point (20°C) which allows a high vapor pressure at standard temperature.
This property of NO2 enables the use of the gas at standard temperature and pressure.
Mechanism of Action:

The biocidal action of this gas involves the degradation of DNA by the nitration of phosphate backbone.

Ozone is a highly reactive gas that is commonly used to sterilize air and water and as a disinfectant for surfaces.
Ozone is a potent oxidizing agent.
High reactivity of ozone allows the removal of waste ozone by converting the ozone into oxygen by passing it through a simple catalyst.
However, because ozone is an unstable and reactive gas, it has to be produced on-site, which limits the use of ozone in different settings.
It is also very hazardous and thus only be used at a concentration of 5ppm, which is 160 times less than that of ethylene oxide.

Liquid sterilization is the process of sterilization which involves the dipping of equipment in the liquid agent to kill all microorganisms and their spores.
Although liquid sterilization is not as effective as gaseous sterilization, it is appropriate in conditions where a low level of contamination is present.
Various liquid chemicals used for liquid sterilization includes the following:

Hydrogen peroxide is a liquid chemical sterilizing agent which is a strong oxidant.
It is useful in the sterilization of heat or temperature-sensitive equipment.
H2O2 has a short sterilization cycle time as these cycles are as short as 28 minutes where ethylene oxide has cycles that are as long as 10-12 hours.
Drawbacks:

Vaporized hydrogen peroxide (VHP) is used to sterilize largely enclosed and sealed areas, such as entire rooms and aircraft interiors.

Glutaraldehyde is an accepted liquid sterilizing agent which requires comparatively long immersion time.
For the removal of all spores, it requires as long as 22 hours of immersion time.
The presence of solid particles further increases the immersion time.
The penetration power is also lower as it takes hours to penetrate a block of tissues.
The use of glutaraldehyde is thus limited to certain surfaces with less contamination.

Hypochlorite solution, which is also called liquid bleach, is another liquid chemical that can be used as a disinfectant.
Submerging devices for a short period in liquid bleach might kill some pathogenic organisms but to reach sterilization submersion for 20-24 hours is required.
It is an oxidizing agent and thus acts by oxidizing organic compounds which results in the modification of proteins in microbes which might ultimately lead to death.
Appropriate concentrations of hypochlorite can be used for the disinfection of workstations and even surfaces to clean blood spills and other liquids.