Nabila Yasmeen

Mathieu Etienne

Sindhu Piyush

Sharma
email: psharma@ichf.edu.pl

Wlodzimierz Kutner
email: wkutner@ichf.edu.pl

Artificial receptors for electrochemical sensing of bacteria

Keywords: Conducting polymer, Molecularly imprinted polymer, Non-conducting polymer, Pathogenic bacteria imprinting

The present review summarizes the molecular imprinting approach to bacteria electrochemical sensing developed over the last five years. Designing artificial molecularly imprinted polymer (MIP) and aptamer receptors for bacteria sensing is more challenging than for analytes of small and even large molecules like proteins. This herein-considered challenge arises from the bacteria's large size and numerous functional groups in bacteria cell walls for which selective complementary sites imprinted in a polymer are puzzling to generate. Moreover, the morphological characterization and recognition mechanism of the MIPs is discussed here in detail.

Introduction

Bacteria are live pathogens capable of transmitting and spreading infections among humans and animals. Infectious diseases are unlike conventional diseases. They spread fast through the air, water, and direct contact with infected individuals. Therefore, their determination must be fast to stop the spreading of disease. Namely, Escherichia coli, Salmonella typhi, Vibrio cholerae, Bacillus cereus, and Pseudomonas aeruginosa are bacteria that fall in the high and moderate risk of infection category [START_REF] Ahmed | Biosensors for whole-cell bacterial detection[END_REF]. For instance, Pseudomonas aeruginosa can make a person seriously ill within 24 h after infection if not adequately treated [START_REF] Kang | Pseudomonas aeruginosa Bacteremia: Risk Factors for Mortality and Influence of Delayed Receipt of Effective Antimicrobial Therapy on Clinical Outcome[END_REF].

The diagnostic facility must quickly and accurately identify microbial agents associated with infectious diseases. Early-stage detection of some high-risk bacterial infections would avoid lengthy treatments and possible death risks.

Microbiology laboratories have been developing suitable procedures to meet this requirement. However, there are several issues with the existing laboratory-based procedures.

Culture-based methods require sample preparation, plating in suitable media, and waiting for appropriate incubation time to get visible colonies. For incubation, these tests require 48 to 72 h [START_REF] Janda | Culture-independent diagnostic testing: have we opened Pandora's box for good?[END_REF]. Therefore, they are time-consuming and laborious [START_REF] Hameed | Conventional and emerging detection techniques for pathogenic bacteria in food science: A review[END_REF][START_REF] Jafari | Entrapment of uropathogenic E. coli cells into ultra-thin sol-gel matrices on gold thin films: A low cost alternative for impedimetric bacteria sensing[END_REF]. Besides calorimetry, polymerase chain reaction (PCR) and enzyme-linked immunological assay (ELISA) techniques are developed [START_REF] Janda | Culture-independent diagnostic testing: have we opened Pandora's box for good?[END_REF]. These techniques provide the desired selectivity in complex sample matrices. Still, they demand trained operators, costly chemicals, and instruments. Moreover, they involve multistep sample preparation [START_REF] Wang | Point-of-care diagnostics for infectious diseases: From methods to devices[END_REF].

Comparatively, electrochemical sensors are more suitable for field analysis [START_REF] Castle | Electrochemical sensors to detect bacterial foodborne pathogens[END_REF]. Now, handheld commercial potentiostats are available, which are much cheaper than corresponding desktop instruments. Combining the electrochemical transduction of an analytical signal with artificial recognition appears appealing toward the selective determination of different bacterial species [START_REF] Castle | Electrochemical sensors to detect bacterial foodborne pathogens[END_REF][START_REF] Amiri | Electrochemical Methodologies for the Detection of Pathogens[END_REF].

Molecular imprinting is a procedure that generates in a polymer matrix artificial receptors complementary in shape, size, and orientation to the binding sites of the target analyte molecules [START_REF] Sharma | Bioinspired intelligent molecularly imprinted polymers for chemosensing: A mini review[END_REF]. Usually, molecularly imprinted polymers (MIPs) are prepared via copolymerization of functional and cross-linker monomers in the presence of the target analyte, first serving as the template. Removal of the template from the MIP results in vacating of molecular cavities capable of recognizing target analyte molecules selectively. The imprinting procedure is now sufficiently developed to incorporate MIPs into lab-on-a-chip [START_REF] Liu | Electrochemical microfluidic chip based on molecular imprinting technique applied for therapeutic drug monitoring[END_REF][START_REF] Ge | Molecularly Imprinted Polymer Grafted Porous Au-Paper Electrode for a Microfluidic Electro-Analytical Origami Device[END_REF] and point-of-care technologies [START_REF] Li | Molecularly imprinted polymer-enhanced biomimetic paperbased analytical devices: A review[END_REF][START_REF] Mustafa | Molecularly imprinted polymers in diagnostics: accessing analytes in biofluids[END_REF]. MIPs are synthetic alternatives to biorecognition systems, e.g., enzymes and antibodies [START_REF] Bui | Fighting Antibiotic-Resistant Bacteria: Promising Strategies Orchestrated by Molecularly Imprinted Polymers[END_REF].

Their potential to overcome the disadvantages of natural recognition systems, including a high cost, low stability, and susceptibility to surrounding changes, like pH or temperature changes, is recognized [START_REF] Sharma | Bioinspired intelligent molecularly imprinted polymers for chemosensing: A mini review[END_REF][START_REF] Bui | Fighting Antibiotic-Resistant Bacteria: Promising Strategies Orchestrated by Molecularly Imprinted Polymers[END_REF][START_REF] Piletsky | Molecularly imprinted polymers for cell recognition[END_REF][START_REF] Iskierko | Molecularly imprinted polymers for separating and sensing of macromolecular compounds and microorganisms[END_REF].

Designing artificial receptors for bacteria is more challenging than for small-molecules and even large-molecules, e.g., proteins [START_REF] Piletsky | Molecularly imprinted polymers for cell recognition[END_REF][START_REF] Iskierko | Molecularly imprinted polymers for separating and sensing of macromolecular compounds and microorganisms[END_REF]. This challenge arises from the bacteria's large size and the numerous functional groups in bacteria cell walls for which selective complementary imprinted sites are difficult to generate [START_REF] Bui | Fighting Antibiotic-Resistant Bacteria: Promising Strategies Orchestrated by Molecularly Imprinted Polymers[END_REF]. Functional monomers should strongly interact with the bacteria's walls for successful imprinting. However, the composition and morphology of these walls can vary considerably, e.g., in the case of Gram-positive and Gram-negative bacteria or depending on the environmental conditions for the same bacteria. The present review critically summarizes strategies developed over the last five years for bacteria determination via molecular imprinting. Moreover, the MIP morphological characterization and bacteria recognition mechanism are discussed in detail.

Bacteria imprinting at interfaces

Bacteria were first successfully imprinted in 1996 [START_REF] Aherne | Bacteria-Mediated Lithography of Polymer Surfaces[END_REF]. Gram-positive bacteria, Listeria monocytogenes, and S. aureus bacteria were imprinted over polymer beads by emulsion polymerization.

This imprinting method was also explored for the imprinting of small molecules [START_REF] Shen | Molecular imprinting in Pickering emulsions: a new insight into molecular recognition in water[END_REF]. However, more reports later described the imprinting of bacteria by Pickering emulsion polymerization [START_REF] Wongkongkatep | Bacteria Interface Pickering Emulsions Stabilized by Self-assembled Bacteria-Chitosan Network[END_REF][START_REF] Xie | Demulsification of Bacteria-Stabilized Pickering Emulsions Using Modified Silica Nanoparticles[END_REF][START_REF] Shen | Bacterial Imprinting at Pickering Emulsion Interfaces[END_REF]. This polymerization led to MIP beads with well-controlled hierarchical structures. The interfacial imprinting was carried out in particle-stabilized oil-in-water emulsions, where the bacteria were present on the surface of nanoparticles during the polymerization of the monomer phase. The oil phase contained cross-linking monomers, and the initiator was in an emulsion.

After mixing the oil phase with an aqueous suspension of the bacteria-monomer complex, a stable emulsion was produced by shaking the mixture vigorously. After polymerization, the (bacteria template)-modified nanoparticles were removed from the new spherical particles to leave tiny indentations decorated with molecularly imprinted cavities.

The pre-polymerization complexes were prepared with a monomer, including cationic chitosan [START_REF] Wongkongkatep | Bacteria Interface Pickering Emulsions Stabilized by Self-assembled Bacteria-Chitosan Network[END_REF][START_REF] Shen | Bacterial Imprinting at Pickering Emulsion Interfaces[END_REF], that interacted with the target bacteria surface groups during this imprinting.

A well-controlled hierarchical structure containing large pores decorated with easily accessible molecular binding sites was synthesized. The binding experiments involving different bacteria strains provided strong evidence that bacteria recognition with the beads depended on the nature of the monomer used to prepare a pre-polymerization complex and the target bacteria.

However, the beads' size exceeded 100 m.

To solve the size issue, Escherichia coli O157:H7 was imprinted over 5-µm diameter spheres in an aqueous medium by the layer-by-layer approach (Figure 1a) [START_REF] Shan | Spontaneous and specific binding of enterohemorrhagic Escherichia coli to overoxidized polypyrrole-coated microspheres[END_REF]. Plastic beads coated with Au film served as the core. On top of this Au film, an aminothiophenol monolayer was assembled.

Then, through electrostatic interactions, Nafion was coated over the assembled monolayer.

Nafion-modified surface played an essential role in grafting imprinting shell-over-core particles.

The MIP shell was prepared by chemical oxidation of pyrrole in an aqueous medium containing E. coli O157:H7. The presence of bacteria in spherical particles was clearly evidenced by fluorescence imaging (Figure 1b).

An interesting report describes MIP film preparation via sol-gel transition-derived methods using functionalized organosilane as the monomer and lipopolysaccharide from Pseudomonas Aeruginosa as the template over kaolin particles. This material was prepared for wastewater purification, particularly focusing on removing Gram-negative and Gram-positive bacteria [START_REF] Gavrila | Molecularly imprinted films and quaternary ammonium functionalized microparticles working in tandem against pathogenic bacteria in wastewaters[END_REF].

Bacteria imprinting in thin films

Bacteria imprinted in nanoparticles are not used in electrochemical sensor fabrication. Instead, imprinting in a thin polymer film is a much-explored approach. In this approach, the cavities imprinted for bacteria are generated on the surface of the polymer film deposited/coated over the transducer surface (Scheme 1a) [START_REF] Jafari | Entrapment of uropathogenic E. coli cells into ultra-thin sol-gel matrices on gold thin films: A low cost alternative for impedimetric bacteria sensing[END_REF][START_REF] Yasmeen | Molecularly imprinted polymer as a synthetic receptor mimic for capacitive impedimetric selective recognition of Escherichia coli K-12[END_REF][START_REF] Jiang | A self-driven PET chip-based imprinted electrochemical sensor for the fast detection of Salmonella[END_REF]. Through this approach, imprints, complementary in shape and size to whole bacteria, were generated. Because of the large bacteria size, their immobilization before polymer deposition and careful tuning of polymer thickness is not mandatory (Scheme 1a). These steps are pivotal for small molecules and protein imprinting. This imprinting results in partial molecule encapsulation [START_REF] Yasmeen | Molecularly imprinted polymer as a synthetic receptor mimic for capacitive impedimetric selective recognition of Escherichia coli K-12[END_REF]. That makes bacteria surface imprinting different from the imprinting of small molecules and proteins.

Surface groups of bacteria play a crucial role in preparing selective cavities in MIP artificial receptors (Scheme 1b). Bacteria imprinting in a conducting polymer involves mostly one monomer for preparing selective cavities. For instance, a monomer containing the OH [START_REF] Khan | Artificial receptors for the electrochemical detection of bacterial flagellar filaments from Proteus mirabilis[END_REF][START_REF] Lin | Dual synthetic receptor-based sandwich electrochemical sensor for highly selective and ultrasensitive detection of pathogenic bacteria at the single-cell level[END_REF], COOH [START_REF] Wang | Rapid, sensitive and label-free detection of pathogenic bacteria using a bacteria-imprinted conducting polymer film-based electrochemical sensor[END_REF], or boronic acid [START_REF] Yasmeen | Molecularly imprinted polymer as a synthetic receptor mimic for capacitive impedimetric selective recognition of Escherichia coli K-12[END_REF][START_REF] Golabi | Electrochemical bacterial detection using poly(3-aminophenylboronic acid)-based imprinted polymer[END_REF][START_REF] Wang | Machine learningassisted cell-imprinted electrochemical impedance sensor for qualitative and quantitative analysis of three bacteria[END_REF] moiety was used to introduce binding sites in cavities.

The boronic acid group covalent binding of Gram-positive bacteria was exploited to differentiate it from Gram-negative bacteria because of the thick peptidoglycan cell wall. At the same time, polypyrrole (which contains N-H interaction sites) appeared efficient for providing desired selectivity [START_REF] Liustrovaite | Towards electrochemical sensor based on molecularly imprinted polypyrrole for the detection of bacteria-Listeria monocytogenes[END_REF][START_REF] Tokonami | Mechanism in external field mediated trapping of bacteria sensitive to nanoscale surface chemical structure[END_REF]. Besides, the imprinted polymer selectivity and binding performance increased if more than one functional monomer was used [START_REF] Akhtarian | Metal Microwires Functionalized with Cell-Imprinted Polymer for Capturing Bacteria in Water[END_REF]. In one such report, an imprinted polymer for E. coli OP50 with 2.2(±0.4) μm thickness was prepared. The bacteria capture efficiency highly depended on the choice of the functional monomer. When the methacrylic acid was used as a functional monomer, the efficiency of the imprinted polymer film-coated wire was the lowest (40%). Adding acrylamide, methyl methacrylate, and N-vinylpyrrolidone as functional monomers significantly increased the bacteria capture efficiency to 76(±10)%, resulting in MIP formation. Further application of this polymer-modified wire has been predicted in the direct electrical detection of bacteria.The involvement of two or more functional monomers is commonly reported when bacteria are imprinted in a photo or thermally polymerizable monomers. Drop coating [START_REF] Divagar | Surface imprinted Ag decorated MnO2 thin film electrodes for the synergic electrochemical detection of bacterial pathogens[END_REF], stamping [START_REF] Werner | Investigations on substructures within cavities of surface imprinted polymers using AFM and PF-QNM[END_REF][START_REF] Brauer | Raman and scanning probe microscopy for differentiating surface imprints of E. coli and B. cereus[END_REF][START_REF] Givanoudi | Selective Campylobacter detection and quantification in poultry: A sensor tool for detecting the cause of a common zoonosis at its source[END_REF][START_REF] Poller | Surface Imprints: Advantageous Application of Ready2use Materials for Bacterial Quartz-Crystal Microbalance Sensors[END_REF], and electro-initiated deposition [START_REF] Golabi | Electrochemical bacterial detection using poly(3-aminophenylboronic acid)-based imprinted polymer[END_REF][START_REF] Butina | Electrochemical sensing of bacteria via secreted redox-active compounds using conducting polymers[END_REF][START_REF] Aparna | Molecularly imprinted conducting polymer based sensor for Salmonella typhimurium detection[END_REF][START_REF] Zhang | Highly stable and reusable imprinted artificial antibody used for in situ detection and disinfection of pathogens[END_REF] are the most common strategies to coat transducer surfaces with MIPs. Chemical immobilization through electrodeposition produced a much more stable film than those immobilized physically [START_REF] Sikolenko | Deposition routes of molecularly imprinted silica for the development of highly specific electrochemical "in-field" sensors[END_REF]. The recognition unit stability is an essential issue for POC device fabrication. Sometimes, a multipleuse system is required.

Bacteria presence and removal from imprints

The presence or templating of bacteria in the polymer is much easier to confirm than small molecules and proteins. Morphological characterization techniques, including scanning electron microscopy (SEM) [START_REF] Aherne | Bacteria-Mediated Lithography of Polymer Surfaces[END_REF][START_REF] Shen | Bacterial Imprinting at Pickering Emulsion Interfaces[END_REF][START_REF] Sen | coli imprinted nano-structured silica micro-granules by spray drying: Optimization of calcination temperature[END_REF][START_REF] Gong | Imprinted Polymer Beads Loaded with Silver Nanoparticles for Antibacterial Applications[END_REF] and atomic force microscopy (AFM), are used to confirm bacteria presence in the polymer [START_REF] Werner | Investigations on substructures within cavities of surface imprinted polymers using AFM and PF-QNM[END_REF][START_REF] Brauer | Raman and scanning probe microscopy for differentiating surface imprints of E. coli and B. cereus[END_REF]. The SEM micrometer-range resolution confirms the presence and absence of bacteria in the polymer.

Similarly, AFM, with its atomic scale resolution, allows the investigation of micro-sized patterning in the polymer. More advanced AFM techniques, e.g., peak force quantitative nanomechanics (PF-QNM), are now used to map adhesion properties [START_REF] Werner | Investigations on substructures within cavities of surface imprinted polymers using AFM and PF-QNM[END_REF][START_REF] Brauer | Raman and scanning probe microscopy for differentiating surface imprints of E. coli and B. cereus[END_REF], as imprinting proposes that functional monomers interact with complementary groups present on the target species. After template removal, cavities containing recognizing sites complementary to the template functionalities in surface chemistry and shape are formed. That results in selective recognition. The PF-QNM was used to compare two imprinting techniques: stamping and Pickering emulsions polymerization. Both imprintings resulted in rod-shaped cavities on the imprinted polymer surfaces. However, the produced imprints distinctly differed from each other.

Stamping imprinting led to cavities comprising smooth surfaces, reflecting the geometry of dried E. coli cells on the stamp. The cavities, produced via polymerization of E. coli stabilized o/w emulsions (Pickering emulsions), had rougher and globular structures because the adhesion properties inside the imprints differed from those outside [START_REF] Werner | Investigations on substructures within cavities of surface imprinted polymers using AFM and PF-QNM[END_REF].

The presence and absence of bacteria templates in imprints were easily distinguished.

However, the bacteria template removal from these imprints is not comparatively straightforward. Enzymes or suitable solution mixtures facilitated the digestion of the bacteria templates through the exposed cell wall surface. Several reports adopted a multistep protocol for bacteria templates' removal from their imprints. Enzyme treatment, washing with surfactants, and the overoxidation of the MIP film were performed consecutively [START_REF] Yasmeen | Molecularly imprinted polymer as a synthetic receptor mimic for capacitive impedimetric selective recognition of Escherichia coli K-12[END_REF][START_REF] Jiang | A self-driven PET chip-based imprinted electrochemical sensor for the fast detection of Salmonella[END_REF][START_REF] Khan | Artificial receptors for the electrochemical detection of bacterial flagellar filaments from Proteus mirabilis[END_REF] or alone [START_REF] Wang | Rapid, sensitive and label-free detection of pathogenic bacteria using a bacteria-imprinted conducting polymer film-based electrochemical sensor[END_REF] to remove the bacteria. Moreover, multiple washing with ethanol-hydrochloric acid (3 M) (5:1, v/v) solution was reported [START_REF] Tajani | Ultra selective and high-capacity dummy template molecularly imprinted polymer to control quorum sensing and biofilm formation of Pseudomonas aeruginosa[END_REF]. This aggressive solution mixture was necessary because bacteria were imprinted in the bulk polymer. Interestingly, when bacteria were imprinted with (phenylboronic acid)-based monomer, a short-time washing with 20 mM fructose sufficed for their removal [START_REF] Golabi | Electrochemical bacterial detection using poly(3-aminophenylboronic acid)-based imprinted polymer[END_REF]. Removing bacteria from the PDMS stamps was easy; short sonication in water [START_REF] Hui | Capture and kill: Selective eradication of target bacteria by a flexible bacteria-imprinted chip[END_REF][START_REF] Ren | Sorting inactivated cells using cell-imprinted polymer thin films[END_REF][START_REF] Ren | Chemical Recognition in Cell-Imprinted Polymers[END_REF] or surfactant washing [START_REF] Arreguin-Campos | Imprinted polydimethylsiloxane-graphene oxide composite receptor for the biomimetic thermal sensing of Escherichia coli[END_REF] successfully removed bacterial cells.

Bacteria electrochemical sensing

A redox probe is often added to the test solution for indirect electrochemical determination of the analyte with an MIP-based chemosensor. Initially, it was speculated that the target analyte molecules binding in molecular cavities caused MIP film swelling. This swelling would change the film permeability to redox probe molecules or ions, thus significantly increasing faradaic currents corresponding to oxidation or reduction of the redox probe. Therefore, this was called the "gate effect" [START_REF] Sharma | Gate effect' in molecularly imprinted polymers: the current state of understanding[END_REF]. This effect is much exploited in fabricating bacteria chemosensors [START_REF] Jiang | A self-driven PET chip-based imprinted electrochemical sensor for the fast detection of Salmonella[END_REF][START_REF] Wang | Rapid, sensitive and label-free detection of pathogenic bacteria using a bacteria-imprinted conducting polymer film-based electrochemical sensor[END_REF][START_REF] Golabi | Electrochemical bacterial detection using poly(3-aminophenylboronic acid)-based imprinted polymer[END_REF][START_REF] Sikolenko | Deposition routes of molecularly imprinted silica for the development of highly specific electrochemical "in-field" sensors[END_REF][START_REF] Shlosberg | Advantages of imprinted polymer electrodes for electrochemical pathogen detection[END_REF]. However, a different sensing mechanism is proposed in this case. Bacteria accumulation in the MIP film pores blocks the redox probe molecules or ions flow through these films to the electrode surface (Scheme 3) [START_REF] Sharma | Gate effect' in molecularly imprinted polymers: the current state of understanding[END_REF]. That results in a decrease in redox probe faradaic current. In the bacteria's absence, unhindered redox probe diffusion results in a pronounced faradaic current. The decrease of current in the presence of bacteria in the polymer was successfully exploited in fabricating different chemosensors.

The imprinted polymer prepared and deposited by electropolymerization has the edge over polymers prepared and deposited differently for sensor fabrication for bacteria. Both conducting (pyrrole, thiophene, aniline derivative) [START_REF] Jiang | A self-driven PET chip-based imprinted electrochemical sensor for the fast detection of Salmonella[END_REF][START_REF] Wang | Rapid, sensitive and label-free detection of pathogenic bacteria using a bacteria-imprinted conducting polymer film-based electrochemical sensor[END_REF][START_REF] Golabi | Electrochemical bacterial detection using poly(3-aminophenylboronic acid)-based imprinted polymer[END_REF][START_REF] Aparna | Molecularly imprinted conducting polymer based sensor for Salmonella typhimurium detection[END_REF] and non-conducting (phenol) [START_REF] Khan | Artificial receptors for the electrochemical detection of bacterial flagellar filaments from Proteus mirabilis[END_REF] polymers are involved in these preparations (Scheme 4). Non-conducting imprinted polymer matrices were prepared using products of sol-gel transition [START_REF] Jafari | Entrapment of uropathogenic E. coli cells into ultra-thin sol-gel matrices on gold thin films: A low cost alternative for impedimetric bacteria sensing[END_REF][START_REF] Sikolenko | Deposition routes of molecularly imprinted silica for the development of highly specific electrochemical "in-field" sensors[END_REF] and polydimethylsiloxane (PDMS) [START_REF] Hui | Capture and kill: Selective eradication of target bacteria by a flexible bacteria-imprinted chip[END_REF]. Recently, examples of imprinting in a sol-gel-transition prepared matrix were summarized in a review article [START_REF] Sikolenko | Deposition routes of molecularly imprinted silica for the development of highly specific electrochemical "in-field" sensors[END_REF]. Even though the sol-gel transition generates a nonconducting matrix, applying this matrix in the fabrication of electrochemical sensors is growing [START_REF] Sikolenko | Deposition routes of molecularly imprinted silica for the development of highly specific electrochemical "in-field" sensors[END_REF]. It is hard to infer whether a functional monomer, which produces conducting or nonconducting polymer, should be preferred for bacteria imprinting because comparative studies are unavailable. Considering sensitivity, conducting polymer-based MIP would be preferable as it can transduce every interaction of bacteria with cavities in the polymer (Scheme 3a). However, at the same time, few such interactions can be non-selective (outside of imprints), resulting in a false analytical signal (Scheme 3a). In contrast, a non-conducting polymer-based MIP can be more selective because a redox probe can exchange charge with the electrode only owing to its diffusion through pores generated in the MIP after bacteria removal. This diffusion will only be blocked if bacteria are bound to its cavities (Scheme 3b). A report described highly selective E. coli imprinting in a non-conducting sol-gel matrix prepared by tetraethoxysilane condensation with (3-mercaptopropyl)trimethoxysilane [START_REF] Jafari | Entrapment of uropathogenic E. coli cells into ultra-thin sol-gel matrices on gold thin films: A low cost alternative for impedimetric bacteria sensing[END_REF]. However, not enough data were available to calculate the selectivity factor. Nevertheless, the conclusion follows from the response shown. The selectivity of the Staphylococcus epidermidis imprinted conducting polyaniline to Staphylococcus epidermidis was only two times higher than to Deinococcus proteolyticus, E. coli, and Streptococcus pneumonia [START_REF] Golabi | Electrochemical bacterial detection using poly(3-aminophenylboronic acid)-based imprinted polymer[END_REF]. The involvement of aptamer is reported for fabricating selective MIP-containing sensing systems for Staphylococcus aureus [START_REF] Lin | Dual synthetic receptor-based sandwich electrochemical sensor for highly selective and ultrasensitive detection of pathogenic bacteria at the single-cell level[END_REF][START_REF] El-Wekil | An innovative dual recognition aptasensor for specific detection of Staphylococcus aureus based on Au/Fe3O4 binary hybrid[END_REF][START_REF] Sarabaegi | Rapid and sensitive determination of Pseudomonas aeruginosa by using a glassy carbon electrode modified with gold nanoparticles and aptamer-imprinted polydopamine[END_REF]. Usually, aptamers are identified by cell-based systematic evolution of ligands by exponential enrichment (SELEX) [START_REF] Chang | Rapid single cell detection of Staphylococcus aureus by aptamer-conjugated gold nanoparticles[END_REF]. The MIP film was deposited on gold nanoparticles at the iron(II, III) oxide particles (AuNPs@Fe3O4) coated electrode in the presence of aptamer-conjugated S. aureus by electropolymerization of the o-phenylenediamine monomer [START_REF] El-Wekil | An innovative dual recognition aptasensor for specific detection of Staphylococcus aureus based on Au/Fe3O4 binary hybrid[END_REF]. A similar approach used polydopamine to imprint S. aureus [START_REF] Sarabaegi | Rapid and sensitive determination of Pseudomonas aeruginosa by using a glassy carbon electrode modified with gold nanoparticles and aptamer-imprinted polydopamine[END_REF]. Because of the selective aptamer recognition unit, the chemosensor determined S. aureus even in complex milk samples at as low a concentration as 10 CFU mL -1 [START_REF] Lin | Dual synthetic receptor-based sandwich electrochemical sensor for highly selective and ultrasensitive detection of pathogenic bacteria at the single-cell level[END_REF]. Moreover, it was possible to selectively measure the S. aureus concentration in a 10-fold diluted sample without pretreatment. This high sensitivity reaching was possible because of the synergy of aptamer and electroactive 6-(ferrocenyl)-hexanethiol co-functionalized gold nanoparticles as the signal probe.

Conclusions

The last five years' literature reveals that artificial receptors prepared with electropolymerizable monomers appeared to be the preferable choice to prepare "artificial receptors" for sensing bacteria. With this approach, in one step, encapsulation of bacteria in conducting or nonconducting polymer film is possible. These systems' sensitivity reaches 2 to 100 CFU mL -1 [START_REF] Jiang | A self-driven PET chip-based imprinted electrochemical sensor for the fast detection of Salmonella[END_REF][START_REF] Lin | Dual synthetic receptor-based sandwich electrochemical sensor for highly selective and ultrasensitive detection of pathogenic bacteria at the single-cell level[END_REF][START_REF] Wang | Rapid, sensitive and label-free detection of pathogenic bacteria using a bacteria-imprinted conducting polymer film-based electrochemical sensor[END_REF][START_REF] Golabi | Electrochemical bacterial detection using poly(3-aminophenylboronic acid)-based imprinted polymer[END_REF]. However, multistep extraction is necessary to remove partially encapsulated bacteria without leaving any debris in imprinted cavities. For future applications, a mild procedure for bacteria removal must be developed. Aggressive extraction solutions can damage microfluidic or POC devices. Bacteria removal from MIPs in the stamping technique is easy and fast. However, the cavities are formed by contacting substrate-immobilized dry cells, which do not necessarily have the same shape as water-swelled bacteria.

Although hundreds of reports described successful imprinting, prepared MIPs primarily determine non-pathogenic strains. There are few attempts to imprint pathogenic strain for fabricating sensors for their determination [START_REF] Lin | Dual synthetic receptor-based sandwich electrochemical sensor for highly selective and ultrasensitive detection of pathogenic bacteria at the single-cell level[END_REF][START_REF] Wang | Rapid, sensitive and label-free detection of pathogenic bacteria using a bacteria-imprinted conducting polymer film-based electrochemical sensor[END_REF][START_REF] Wang | Reusable and universal impedimetric sensing platform for the rapid and sensitive detection of pathogenic bacteria based on bacteria-imprinted polythiophene film[END_REF][START_REF] Bezdekova | Magnetic molecularly imprinted polymers used for selective isolation and detection of Staphylococcus aureus[END_REF]. Moreover, all reported bacteria determinations are indirect, involving an electrochemical response of a redox probe. Taking advantage of the electroactive nature of bacteria for their determination would be a significant step forward. The application of bacteria-imprinted microparticles in chemosensing requires further exploration.

Figure 1 .

 1 Figure 1. (a) Schematic illustration of the bacteria imprinting on the surface of the polypyrrole shell deposited on the Nafion/Au film-coated plastic microparticle core. (b) SEM and fluorescence images of the microsphere coated with the E. coli-doped polypyrrole. Adapted with permission from [22].

Scheme 1 .

 1 Scheme 1. Illustration of approaches used to prepare a bacteria-imprinted polymer film. (a) The bacteria-imprinted polymer film deposited by electropolymerization. (b) The bacteria-imprinted film prepared after engaging the surface groups of bacteria.

Scheme 2 .

 2 Scheme 2. (Center) The "gate effect" mechanism showing physical pore blocking by bacteria that hinders redox probe diffusion through the MIP film to the electrode surface. (Left) The EIS and (right) DPV signal in (red curve) the presence and (green curve) absence of bacteria physical blocking of redox probe oxidation at an MIP film-coated electrode.

Scheme 3 .

 3 Scheme 3. Electrochemical signals originating from bacteria's physical blocking of the redox probes' charge exchange with the (a) conducting and (b) non-conducting MIP film-coated electrode.

Acknowledgments P.S.S. thanks to the National Science Centre of Poland, grant NCN 2017/25/B/ST4/01696, for financial support.

Abbreviations

AFM, atomic force microscopy; AuNP, gold nanoparticle; ELISA, enzyme-linked immunological assay; MIP, molecularly imprinted polymer; PF-QNM, peak force quantitative nanomechanics; PDMS, polydimethylsiloxane; PCR, polymerase chain reaction; POC, point-of-care (device); SELEX, systematic evolution of exponential enrichment; SEM, scanning electron microscopy