407
Fundamental approaches for Cleaning and
antimicrobial actions
Needs for achieving clean and hygienic surfaces have been with us
for a long time, and many conventional material responses to them
have achieved varying degrees of success. The use of nanomaterials
for self-cleaning, antimicrobial, fungicidal, and related applications
for various types of surfaces is more recent but has quickly become
widespread. Most nano-based products intended for these applications use coatings, paints, or films that contain nanoparticles
and that are applied or bonded to conventional materials. Some
applications, however, have surface layers in which nanoparticles
have been directly intermixed into the base material. Though many
needs, such as cleaning or antifungal, may initially seem quite different, technological responses are often quite similar.
Three primary technologies drive applications in these areas: the
ability to make hydrophobic (water-repelling) surfaces, hydrophilic
(water-attracting) surfaces, and photocatalytic surfaces. Nano-based
approaches that invoke hydrophilic (water-repelling) action to
spread water or hydrophobic action to bead water are widely used
in achieving clean surfaces (see Figure 10.2). Nanomaterials with
special photocatalytic properties that absorb ultraviolet (UV) energy
that in turn causes dirt or other foreign particles on the surface to
slowly loosen or break down via oxidation can aid in self-cleaning
and can provide antimicrobial action. As we will see, actions such
as self-cleaning can be accomplished in several ways and can be
based on hydrophobic, hydrophilic, or photocatalytic means; these
primary approaches can be combined in a variety of ways. Antimicrobial action can also be achieved via photocatalytic action or
by making direct use of embedded nanomaterials such as copper
that have intrinsic anti-microbial action, as we’ll discuss later.
Prior to looking at specific applications in detail, it is useful to look
at a common referent in nature to understand how certain types
of cleaning actions can naturally occur. The lotus flower has long
been a symbol of purity in Asian countries (see Figure 10.3). In
the 1970s, two botanical researchers (Barthlott and Neinhuis, from
the University of Bonn) noticed that the surfaces of the flower are
rarely soiled, even as they arise from muddy lands, and that this
effect could be observed in many plants that are covered in tiny wax
structures that appeared to make the surfaces unwettable. Interestingly, using a scanning electron microscope (SEM) they found that
the surfaces were not smooth. Conventional wisdom suggested that
the smoother the surface, the less likely dirt was to adhere to it,
Hydrophobic
(water repelling; drops form beads)
Hydrophilic
(water attracting; drops flatten out)
Photocatalytic
(UV-induced reactions that cause decomposition of dirt molecules)
(a) Basic actions
(b) Water drop on hydrophobic surface
Figure 10.2
Basic approaches for self-cleaning, easy-cleaning,
and antimicrobial functions. Some nanomaterials,
e.g., copper, have intrinsic antimicrobial actions
as well.
Functional Characteristics
Fundamental approaches for Cleaning and
antimicrobial actions
Needs for achieving clean and hygienic surfaces have been with us
for a long time, and many conventional material responses to them
have achieved varying degrees of success. The use of nanomaterials
for self-cleaning, antimicrobial, fungicidal, and related applications
for various types of surfaces is more recent but has quickly become
widespread. Most nano-based products intended for these applications use coatings, paints, or films that contain nanoparticles
and that are applied or bonded to conventional materials. Some
applications, however, have surface layers in which nanoparticles
have been directly intermixed into the base material. Though many
needs, such as cleaning or antifungal, may initially seem quite different, technological responses are often quite similar.
Three primary technologies drive applications in these areas: the
ability to make hydrophobic (water-repelling) surfaces, hydrophilic
(water-attracting) surfaces, and photocatalytic surfaces. Nano-based
approaches that invoke hydrophilic (water-repelling) action to
spread water or hydrophobic action to bead water are widely used
in achieving clean surfaces (see Figure 10.2). Nanomaterials with
special photocatalytic properties that absorb ultraviolet (UV) energy
that in turn causes dirt or other foreign particles on the surface to
slowly loosen or break down via oxidation can aid in self-cleaning
and can provide antimicrobial action. As we will see, actions such
as self-cleaning can be accomplished in several ways and can be
based on hydrophobic, hydrophilic, or photocatalytic means; these
primary approaches can be combined in a variety of ways. Antimicrobial action can also be achieved via photocatalytic action or
by making direct use of embedded nanomaterials such as copper
that have intrinsic anti-microbial action, as we’ll discuss later.
Prior to looking at specific applications in detail, it is useful to look
at a common referent in nature to understand how certain types
of cleaning actions can naturally occur. The lotus flower has long
been a symbol of purity in Asian countries (see Figure 10.3). In
the 1970s, two botanical researchers (Barthlott and Neinhuis, from
the University of Bonn) noticed that the surfaces of the flower are
rarely soiled, even as they arise from muddy lands, and that this
effect could be observed in many plants that are covered in tiny wax
structures that appeared to make the surfaces unwettable. Interestingly, using a scanning electron microscope (SEM) they found that
the surfaces were not smooth. Conventional wisdom suggested that
the smoother the surface, the less likely dirt was to adhere to it,
Hydrophobic
(water repelling; drops form beads)
Hydrophilic
(water attracting; drops flatten out)
Photocatalytic
(UV-induced reactions that cause decomposition of dirt molecules)
(a) Basic actions
(b) Water drop on hydrophobic surface
Figure 10.2
Basic approaches for self-cleaning, easy-cleaning,
and antimicrobial functions. Some nanomaterials,
e.g., copper, have intrinsic antimicrobial actions
as well.
Functional Characteristics