Surface functionalization is an impartment of new properties to a surface through partial or complete alteration of its chemical functional groups. Methods of surface modification are abundant and extensively used in modern technology for controlling wettability, increase of corrosion resistance, biological compatibility, and change of mechanical properties. Apart from bulk functionalization, these methods have also found a wide application in nanotechnology in the process of formation of micro- and nanoscale structures and patterns . All the existing strategies of local surface properties alteration can be divided into two categories according to whether or not a mask is used in the technological process. Obviously, in mask methods alteration of functional groups is done with a patterned resistant mask, while in mask-free methods the mask is absent. MASK METHODS Mask patterning technologies (Fig.1) are widely used in formation of nanoelectronic and photonic elements, nano- and microelectromechanical systems, microfluidics, production of nanostructures and other functional nanoelements. High productivity, reproducibility, and possibility of scaling make mask patterning methods a useful tool in applied science and manufacture.
Photolithography Combination of photolithography with photosensitive chemical reactions is one of the most evident examples of the template use for the surface functionalization. A particular case of such approach is modification of silicon surface with derived olefins. In this case a precursor molecule demands UV activation for interaction with the surface, thus, the use of masks permits to create a monolayer of organic molecules exactly in the points of UV exposure . Subsequently this monolayer can be used as a base for construction of more complex functional surfaces [3, 4].
A technique for attachment and removal of photosensitive protective groups is another example of photolithographic surface functionalization . The most common method is formation of self-assembling film containing a functional group that further undergoes photochemical transformation [5, 6]. A thiol-ene addition reaction that belongs to click chemistry is prospective for high-yield creation of complex organic blocks on the surface . This reaction can be combined well with photochemical deposition of alkenes onto reduced silicon surface through the use of phototemplates . The surface in this case can be preliminary modified either with diene or thiol functional groups; for the latter the pattern is formed via phototemplate from functional block containing diene group . Click chemistry permits high-yield immobilization of different biomolecules  and cells , polymer surface functionalization , and creation of specific sorption sites for quantum dots . Electrochemical patterning Aside from photosensitive reactions, templates can be used for surface patterning by means of electrochemical reactions. In this case metallic template acts as one of the electrodes on which electrochemical reaction is conducted. Such microlithography technique allows to perform electrochemical oxidation of top methyl group of silane self-assembling film preserving its high-ordered structure . Thus, electrochemical microlithography can be applied to creation of hydrophilic patterns on hydrophobic surface.
High density electrode array can act as phototemplate making electrochemical reaction on electrode nearby the surface to remove the protective group. This approach is used in solid state DNA synthesis on DNA chips . If the surface itself can act as a second electrode, the use of electrode array permits to develop different strategies based on common electrode functionalization procedures . Microcontact printing Transfer of functional groups and molecules via contact printing is another variant of the template use. Technology of microcontact printing that belongs to a wider class of soft lithography methods has gotten extensive application to formation of functional micropatterns on the surface. Microcontact printing was initially developed in early nineties  to transfer thiol-based self-assembling monolayers onto gold surface; however, further this technology was advanced for patterning with different compounds on different types of surfaces . Firstly, microcontact printing demands formation of a soft elastomeric stamp (mostly made of polydimethylsiloxane, PMDS) that will be used as a base for contact printing. Then the stamp is immersed into a solution of compounds being transferred. Due to capillary action part of the solution remains absorbed on the surface of the stamp after its subsequent ejection and ideally is transferred to a target surface in a contact point between the stamp and the target surface. Nowadays microcontact printing can be applied to transfer almost any material, e.g. metals , polymers , biomolecules , different nanostructures [22, 23].
MASK-FREE METHODS The use of templates gives an opportunity for fast drawing of the same pattern on a relatively large area. However, it’s necessary to notice that functionalization process is dependent on a particular template. Template formation technologies require expensive high tech equipment, and the use of one standard operation per template made. This kind of process is suitable for making patterns with properties opposite to those of the target surface, such as formation of a hydrophilic pattern on a hydrophobic surface , or creation of areas with high adhesion to cells for their fixation in predefined location within a flow chamber . The use of templates for generation of multifunctional surfaces (such as development of multisensory chemical detection systems) or fast versatile prototyping seems less effective.
An alternative group represents a group of mask-free methods called direct writhing methods (Fig.2). These methods allow alteration of group composition in certain location of target surface by local treatment. They have low productivity, but often possess great versatility in regards to both choices of available surfaces and compounds that could be used as functional blocks. Due to their simplicity and versatility mask-free methods got wide application in academic environment in research of different effects in nanodevices and single intermolecular interactions. Inkjet printing Contactless printing implies absence of mechanical contact between print head and substrate during transfer of functional material. Instead material is precisely supplied through a nozzle placed directly over a target area on a distance of 1–5 mm. Printers used for surface functionalization are often adapted for printing with demanded compound instead of conventional inks. Inkjet transfer supposes extrusion of certain volume from the print head through the nozzle. Printers can be divided into thermal, piezoelectric, acoustic, electrostatic, and electrodynamic according to the way of pressure generation in the print head. Currently, thanks to its low price and simplicity, inkjet printing is popular in making patterns from polymeric materials , inorganic particles , crystal films for microelectronic purposes , and chemicals ; it is also widely used in development and manufacture of biosensors [29–31]. Besides, modern inkjet printing methods have shown capability to print with intact cells and form intact tissues .
Electronic beam lithography Electronic beam lithography is a subtype of mask-free nanolithography extensively used for nanoscale patterning . Similarly to photolithography, exposure of organic monolayer on the surface to the electron beam initiates its polymerization altering its resistance to solvents. Apart from polymerization initiation, electron beam exposure of local area allows to remove certain functional groups (such as amino  or polyethylene glycol  groups) from the surface with resolution of several nanometers. Exposed areas can be further used for addition of functional blocks, for example, immobilization of different biomolecules . Under optimized conditions electron beam lithography permits obtaining nanostructures with lateral resolution of less than 5 nm .
Nanolithography with probe microscopy Surface functionalization methods with the aid of probe microscopes represent a large family of techniques that allows patterning with the resolution of less than 10 nm; those can be divided into two groups.
The first group, called dip-pen nanolithography, uses the probe as an analog of a print head, and is based on transfer of compounds directly from the probe to the target surface. Transfer process is similar to a contact print; during the contact between the probe and the surface capillary forces form a meniscus, through which the transfer goes often followed by a chemical reaction. As for microcontact printing, initially an applicability of the probe for printing was shown for thiol-based self-assembling monolayers on gold surface . Printing potential was dramatically widened by introduction of probe temperature regulation systems for deposition control by melting the target material on the edge of the probe . The second group is based on mechanical, thermal, or electrochemical exposure in the contact point between the probe and the surface. Mechanical action implies removal of the material from the surface by the probe, and is usually applied to soft materials. Probe heating can be used in several ways. Firstly, heating can cause softening of the material and ease of its mechanical removal from the surface . Besides, local heating can activate chemical reaction, such as reaction of change of hydrophobic groups to hydrophilic ones  and thermal reduction of graphene in fabrication of nanoelectronic devices . The most common electrochemical reaction for probe microscopy is anodic oxidation of the surface. The meniscus formed between the probe and the surface acts as a nanoscale electrochemical cell, where conductive probe plays the role of cathode and the surface represents anode on which the oxidation reaction occurs . Popularity of this method is caused by availability of equipment and wide spectrum of substrate materials such as metals, semiconductors, carbon nanomaterials, polymers, and self-assembling films . At the same time the method is versatile since it doesn’t require a resist in formation of nanostructures, because the insulator formed can act as a mask on subsequent etching stage. Apart from that, the oxide generated can be a suitable platform for further functionalization via silane self-assembling films. CONCLUSIONS Despite a wide variety of methods to form patterns on a surface, for some applications, such as production of arrays of nanostructures with different receptors for medical diagnosis, effective solutions currently do not exist. Thus, the development of new high-performance methods of local functionalization remains an important challenge for the nanotechnology industry. ■
The project was supported by the Russian Ministry of Education (project № 16.535.2016 / БЧ)