Introduction

Biosensors and bioassays can replace many of the classical analytical methods such as chromatography, photometry, or mass spectrometry. The most commonly used bio-receptor elements in the design of biosensors are enzymes, antibodies and DNA aptamers [1]. However, some novel bio-recognition elements such as modified enzymes, nanobodies, peptides, affibody, protein scaffolds and DNAzymes have also been recently integrated in the design of biosensors [2]. Despite much progress, increased interest in exploration of novel bio-receptors applicable in sensing methodologies to fulfill the necessity of control specific and cost effective assay still exists. In this context, significant work has been done in exploration of enzyme mimetics to replace natural enzymes in the construction of enzymatic assays. Some level of success has been achieved in this direction with the help of chemically synthesized alternatives. Among enzyme mimetics, non-natural ribosomes and deoxyribosomes have been developed through in vitro selection processes [3]. Other enzyme mimetics including cytochrome P450 mimetic [4], serine proteases mimetics [5], dioxygenase [6], phosphodiesterase [7], ligase [8], nuclease [9] and methanogenesis [10] mimetics have been explored. Alternative to natural and chemical bio-recognition element, nano-receptors have been emerged as a promising receptor material in detection systems for laboratory and field-based applications with the advent of nanotechnology. Recently, nanomaterials have attracted tremendous interest in enzyme mimetic research because of their several distinct properties, such as high surface area to volume ratio, abundance of reactive groups on their surface, unique optical properties and fascinating catalytic activity. To date, catalase-, oxidase-, peroxidase-like activities have been demonstrated for various types of nanomaterials [11]. These nanomaterial-based enzyme mimetics offer advantages of low cost, high stability and sustained catalytic activities. As a consequence, biosensors based on these nanomaterials have been extensively employed for many applications in the bioassay, biotechnology and biomedical field.

With the advent of novel nanostructures and new interface materials, these nano-recognition elements will be major players in future sensors development. Similarly, important advances have been achieved with utilization of nanomaterials in the design of electrochemical sensors to improve their analytical performance. Electrochemical biosensors have been traditionally based on bio-receptor elements for highly specific and selective detection of various analytes, while direct electrochemical detection of electroactive analytes is also possible on transducer surface. However, direct electrochemistry based on redox properties of analyte has poor selectivity, sensitivity and accuracy. To overcome these limitations, analyte specific nanomaterials are attracting increasing interest, thanks to their low cost, simple operation, high sensitivity and fast response [12]. The analyte specific nanomaterials permit a good orientation of moieties onto the electrode surface, decreases distance between electro-active species and transducer surface, thus improving electron transfer rate. Keeping in view the important role of nanomaterials in (bio) sensors design, this review paper will focus on the analytical potential of biomimetic and analyte specific nanomaterials. We will discuss different types of nanomaterials according to their intrinsic nature, detection methodologies employed in non-enzymatic assays and applications of non-enzymatic sensors in the biomedical, environmental and food analysis fields.

Biomimetic nanomaterials

In this section, we will review numerous biomimetic nanomaterials that can replace natural enzymes for sensing applications. Among them, cerium oxide (ceria) nanoparticles are famous for its high catalytic performance in several applications owing to the presence of both Ce+3 and Ce+4 oxidation states and in the presence of oxygen vacancies [11, 13, 14]. The presence of this redox couple and switching of ceria oxidation states (between Ce3+ and Ce4+) makes it a promising material for the fabrication of amperometric biosensors [15]. Nanoceria have been found a promising material for enzyme free detection of different analytes [12]. Wang et al. [16] studied the determination of hydrogen peroxide, glucose and cholesterol by using the respective oxidase and hydrogen peroxide formation with Cu2(OH)3Cl-CeO2 nanocomposite. Iron oxide nanomaterials have also found many applications such as in hyperthermia treatment of cancer [17], targeted drug delivery [18], contrast agents in magnetic resonance imaging [19], magnetic storage media magnetic inks, capture and separation of analysts, biosensing applications etc. [20]. They have a potential to be used in developing different point of care (bio) sensors. Gao et al. [21] unexpectedly discovered the peroxidase like properties of iron oxide magnetic nanoparticles. Since then, a lot of work has been done to explore the biomimetic nature of these iron oxide magnetic nanoparticles. Peroxides play an important role in defending against pathogens and detoxification of reactive oxygen species by catalyzing the oxidation of substrates with peroxides. They used nanoparticles of different sizes to study the effect of particles size on activity of these nanoparticles, and concluded that lower sized nanoparticles showed better response than larger ones. Similarly, manganese dioxide materials with different structural morphologies such as nanowires, nanosheets, nanospheres etc. have been used for their enzyme like properties [22]. Soundappan et al. [23] fabricated manganese oxide electrochemically on indium tin oxide glass electrode for the detection of hydrogen peroxide [24]. Other metal oxides such as zinc oxide [25], copper oxide [26], vanadium oxide [27], AgVO3 [28], cobalt oxide [29] nanomaterials have been reported for showing intrinsic enzyme like properties. Furthermore, metal sulfides such as magnetic Fe3S4 nanoparticles and molybdenum nanosheets have shown peroxidase like activity for the detection of glucose and mercury ions respectively [30, 31].

In addition to metal oxide nanoparticles, metal nanoparticles also have been investigated extensively for their inherent enzyme mimic actions. Gold nanoparticles (AuNPs) have been widely studied for catalytic and biomedical applications due to their excellent biocompatibility, unique chemical, physical, electronic properties, high density and high surface to volume ratio [11]. Comotti et al. [32] studied citrate-coated gold nanoparticles catalyzed aerobic oxidation of glucose. Their proposed mechanism was similar to natural enzymatic activity i.e. glucose oxidase. Li et al. [33] used mercury coated gold nanoparticles as peroxidase mimic for the determination of metallothioneins. Likewise, some other metal nanoparticles including platinum [34] and silver [11] have also been studied as enzyme mimics.

Carbon based nanomaterials, such as graphene [35], fullerene [36], carbon nanotubes [37] have received considerable attention as enzyme mimetic in biosensors. The significant attention towards the application of carbon nanomaterials for biosensors is due to their ability to improve electron-transfer kinetics, high surface-to-volume ratios and biocompatibility associated with their unique morphology. Fullerenes and their derivatives, since their discovery, have been used in various applications such as drug delivery, nanomedicines diagnostics etc., due to their distinctive chemical reactivity and being water insoluble nature. Fullerene has been modified with different water soluble functional groups to make it soluble in water and to be able to use for biomedical applications and exploring enzyme like activities [38]. Similarly, the intrinsic enzyme like activity of single walled carbon nanotubes and multi-walled carbon nanotubes has been reported by many groups [39–42]. The activity of these single walled and multiple walled carbon nanotubes depends upon their surface area, pH, and temperature. Graphene oxide as a promising carbon material was also found to retain intrinsic enzyme like properties as reported by Qu et al. [43]. Chen et al. [44] used reduced graphene oxide nanosheets, functionalized with poly(styrene sulfonate) and used for the colorimetric determination of ascorbic acid. In the same context, next section will focus on the analyte specific nano-materials for various analytes.

Generally, metals are good conductors and hence have low limit of detection. However, they are less stable because of the surface poising due to surface oxidation. Also, metals like platinum, gold, silver etc. are very expensive and the synthesis of their nanoparticles is a complicated procedure. Metal oxide nanoparticles are emerging in the field of biosensors during recent years due to their less complicated synthesis, more stability, less expensive and easy availability [45]. However, they are not very sensitive. To increase their sensitivity, they are to be doped with other metal and non-metal dopants. Carbon materials on the other hand are being non-metallic in nature have high conductivity. The study of their biosensing properties is hot topic in biosensors due to their ease of functionalization, less expensive, highly stable and non-metallic nature which is less poisonous as compared to metals and metal oxides.

Substantial developments have been achieved in synthesis procedures in producing diverse nanomaterials with highly controllable shape, size, surface charge and physicochemical properties [46–48]. The introduction of such nanomaterials has greatly enhanced the sensitivity and selectivity for the direct detection of various analytes, without requirement of any additional dye. Over the past few decades, researchers have synthesized a huge number of nano-materials which have been employed for direct recognition of various analytes on transducer platform.

For example, carbon nanomaterials (CNTs) exhibit numerous fascinating features, which make them suitable for developing electrode materials [49–52]. Peng and co-workers [53] highlighted the functionalization of carbon nanotubes with polyethylenimine for electrochemical detection of chromium (VI) in the concentration range from 0.002–20 μmol L−1, with a detection limit of 0.0006 μmol L−1. CNTs have also been used for electrochemical detection of guanine [54] heavy metals [55], fungicide [56] pharmaceutical formulations [57, 58], phenolic compounds [59, 60] hydrazine [61], nitric oxide [62] and explosives [63]. Similarly, graphene and its derivatives such as graphene oxide, functionalized and doped graphene materials, have also been widely employed for direct recognition of different analytes, as an alternative to direct detection based on the natural enzyme recognition. These materials show unique electronic properties, high chemical stability, large surface-to-volume ratio, good mechanical strength, wide electrochemical window, and excellent support substrate. Until now, graphene have extensively been explored as electrochemical sensors for the direct detection of glucose [64–67], hydrogen peroxide [68–71], neurotransmitters [72] and estriol [73]. Single-walled carbon nanohorns (SWCNHs) is yet another emerging class of carbon-based nanomaterials. They possess a conical shape with a tubular length of ~50 nm and base diameter of ~5 nm. SWCNHs provided good results when employed for the electrochemical assays of different analytes such as hydrazine [74, 75], hydrogen peroxide [75] and bisphenol A [76].

Silica-based materials have strong inorganic solids backbone having large surface area and a three-dimensional structure constitute of highly interconnected open spaces, resulting to generate highly porous structure materials. A variety of possible chemical modifications can be performed on its surface due to its porous nature, which leads to a variety of materials with diverse functionalities and properties [77–79]. Rahim et al. [80] reported in situ synthesis of transition metal phthalocyanines (MPcs) in pores of SiO2/C mesoporous materials. These hybrid materials SiO2/C/MPcs were used as pressed disks to construct carbon ceramic electrode (working electrode) and tested as electrochemical sensor for analyte specific detection of nitrite, dissolved oxygen [81, 82], dopamine [83], ascorbic acid [84], 4-aminophenole [85], oxalic acid [86], and fenitrothion pesticide [87]. Silica based materials were also modified with organic groups, ionic liquids, metal and metal oxides, tested as analytes specific materials for detection of NADH [88, 89], nitrite [90], phenolic compounds [91], dopamine [92] and hydrogen peroxide [93]. These hybrid nanomaterials exhibited excellent activity towards specific detection of target analytes as compared to those based on single component nanomaterial. The composite materials enhance the fast electron transfer involved during redox process. A brief overview on different types of biomimetic nano-materials is provided in the Table 1.

Table 1 An overview on materials with enzyme mimicking properties
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The next section of the review paper will discuss the methodologies to integrate these biomimetic nanomaterials in the development of enzyme mimicking optical or electrochemical assays.

Electrochemical and optical detection using enzyme mimicking nanomaterials

Electrochemical detection

Electrochemical transducers are most widely employed in developing bio/chemo-sensors for various analytes because of its simple fabrication, low-cost, sensitivity, portability, and easy-to-use features [116]. In electrochemical sensors, electrochemical reactions are monitored by measuring a change in current (amperometry), change in potential (potentiometry), change in conduction between electrodes (conductometry), or both resistance and reactance (electrochemical impedance spectroscopy) [46, 117]. Thus, electrochemical sensor devices are commonly divided into three main classes based on the parameter to be measured such as current, potential, and impedance. The detection device based on electrochemical principle, which demonstrates three prime components: (1) analyte, (2) transducer, and (3) signal processing unit.

Electrochemical detection based analytical methods have wedged various fields, comprising environmental analysis, diagnostics, food sciences, pharmacology and enzymatic kinetics [118–120]. Recently, the exploration of more effective transfer of electron between the enzyme active site and the surface of electrode helps to deduce more sensitivity to the biosensors. Furthermore, the enzyme presented low stability with time, pH and temperature has given motivation to explore modifications in the electrochemical biosensors designs. The new substitute is based on to utilize artificial enzymes [121–124] and biomimetic materials [125–127] that attempts to imitate natural enzymes with the same efficiency and selectivity, which can be used to fabricate electrochemical sensors and biosensors with higher sensitivity. In such devices, the electrode surface will be modified by the immobilization of redox substance which act as an enzyme active site that catalyzes the substrate conversion in the same manner. This idea has been described in the literature [83, 92, 128–131], however, still exists a great area to be explored.

Non-enzymatic electrocatalysts exist in various forms, pure metals, alloys and bimetallic system, metal metal-oxide heterogeneous catalyst, metal phthalocyanine and porphyrin complexes, carbon-based materials and boron diamond. All these materials except the last two rely on the transition metal. The process of electrocatalysis usually occur via the adsorption of the analyte to the surface of electrode, which probably encompasses the formation of appropriate bonds between the analyte and the d-electrons and the d-orbitals of the metal substrate [132]. This bond formation and breaking is essential in the midst of catalytic process, the bond of intermediate strength would be ideal and so as not limited to adsorption or hamper adsorption at any instant. The strength of bond is dependent on the Gibbs free energy of adsorption. Otherwise a change in bond strength may be execute an alteration in the metal oxidation state, which may change the metal-adsorbate interaction and boost desorption of the product [132].

Burke [133] suggested a model known as ‘Incipient Hydrous Oxide Adatom Mediator’ (IHOAM) for the electrooxidation of different molecules on metal surface. This model depends on the surveillance that ‘active’ metal surface atoms underwent a pre-monolayer oxidation phase that forms an incipient hydrous oxide layer of reactive OHads, which arbitrate oxidation and hinder reduction of kinetically sluggish electrode reactions. The active sites on the electrode surface have low lattice coordination value and lattice stabilization energy.

Owing to the low stability they are highly reactive, which go through pre-monolayer oxidation at a lower potential than thermodynamic surface oxidation products. The catalytic significance of the active OHads layer was well known as compared to small organic compound oxidation, and the emergence of the hydrous species was identified as a fast, pre-oxidation step succeeding chemisorption of the glucose molecule. The hydrous pre-monolayer then mediates oxidation of the adsorbed species at a lower potential. Agnihotri et al. [134] fabricated non-enzymatic sensor for cholesterol using β-cyclodextrin functionalized graphene. Graphene wrapped Cu2O nanocubes were synthesized by Wei et al. [135] can mimic both oxidase and peroxidase like activity and used for electrochemical detection of glucose and hydrogen peroxide with detection limit of 3.3 μM and 20.8 μM, respectively. Dashtestani et al. [136] fabricated amperometric sensor for superoxide anions based on nanocomposite consisting of gold nanoparticles and the copper (II) complex of cysteine (GNP/Cu-Cys) which mimic the enzyme superoxide dismutase (SOD), with a linear range from 3.1 to 326 μM, a detection limit of 2.8 μM and a sensitivity of 0.018 μA μM cm−2.

Metal nanomaterials such as gold [110, 137, 138], platinum [139, 140], nickle [141–143], palladium [144, 145], and a variety of bimetal nanomaterials [146–149] are potential candidates to mimic natural enzymes. Many metal oxide-based nanomaterials such as cobalt oxide [150], cerium oxide [151, 152], copper oxide [97, 153, 154], manganese oxide [155, 156], vanadium pentoxide [27], and iron oxide [157] have also been explored to imitate natural enzyme. Some metal complexes such as metallophthalocyanines [83, 84] and metal Schiff-base complexes [92] were also reported to exhibit tremendous electrocatalytic activities toward oxidation of dopamine which mimetize dopamine monooxygenase enzyme. The electrochemical detection methods have advantages over the other existing techniques due to its sensitivity, fast response, negligible interference effect and robustness. Different nanomaterials were used as an enzyme mimics for electrochemical assays of different analytes are summarized Table 2. In the upcoming section we will discuss the colorimetric detection techniques by using various nano-materials as enzyme mimetics.

Table 2 An overview on nanoparticle based electrochemical assays using enzyme mimics
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Colorimetric detection

Optical sensing methods have gathered great interest due to their intrinsic high sensitivity and simplicity. Compared to other analytical techniques for the detection of analytes, the colorimetric methods are much simpler and their response can be detected with bare eyes or by photometry [162].

Metal nanoparticles produce an intense absorption due to the movement of conduction band electron when these are excited with electromagnetic waves. In noble metals these localized electrons results in high absorption and scattering which lies in UV-Visible region making them perfect candidate for the colorimetric detections and applications. Also, aggregation of the metal nanoparticles leads to significant change in the color as compared to their color in dispersed state. Most of the metal based colorimetric biosensors are based on mechanism of changing color because of coupling phenomenon due to aggregation of nanoparticles. The change in color may also be due to oxidation/reduction reaction between nanoparticles and analyte [163]. Zhao et al. [164] synthesized modified molybdenum sulphide nanoparticles and fabricated a biosensor for colorimetric detection of hydrogen peroxide and glucose. They presented that cation surfactant modified molybdenum sulphide nanoparticles showed better affinity with positively charged 3,3,5,5, tetramethylbenzidine (TMB). The intensity of the absorption of TMB was increased at 652 nm as concentration of hydrogen peroxide increased in the sample. Similarly, concentration of glucose also changed color intensity of TMB in presence of surfactant modified molybdenum nanoparticles. Cao et al. [165] prepared horseradish peroxidase (HRP) functionalized gold nanoclusters for the colorimetric detection of melamine triggered by visible light at very low concentrations. Similarly, Wang et al. [166] synthesized magnetic mesoporous silica nanoparticles for detection of hydrogen peroxide and glucose calorimetrically through TMB. The magnetic silica nanoparticles oxidized glucose by oxygen to gluconic acid and hydrogen peroxide. This hydrogen peroxide further reacted with TMB which results in color change of solution.

Fluorescent detection

Recently, fluorescent based systems sparked tremendous interests in biosensing field, which in general consist of a recognition module for the target analyte and a fluorescent transducer (normally a fluorescent dye) that produces intensity or wavelength changes of the fluorescent signals on interaction of analyte with recognition element [167]. Recent advancements in the field of nanomaterials have resolved the problem of lower quantum yield and extinction coefficient, by developing nanomaterials having unique optical properties. Variety of nanomaterials, as synthetic fluorophores, have been used as powerful alternative transducers (enzyme mimics) in fluorescent biosensors with an emphasis on improved signal strength and detection limits. Hu et al. reported catalyzed decomposition of hydrogen peroxide by cupric oxide performing peroxidase like activity with a highly fluorescent product [98]. Moreover, glucose or L-lactate were also catalyzed for their detection by cupric oxide nanoparticles with improved detection limits. Shi et al. developed Fe3O4 magnetic microspheres based fluorescent system for the detection of Glucose and p-nitrophenol in human serum and water samples, respectively to mimic peroxidase like activity [168]. The analytes were detected via quantitative measurement of fluorescence production with lower detection limit high sensitivity, good selectivity. The developed system had peachy potential in the sensing and biosensing of variety of H2O2 based analytes. Similarly, Qiu’s group utilized cerium coordinated polymer nanoparticles as artificial peroxidase for selective and sensitive detection of hydrogen peroxide [169]. Free radical scavenging property of these cerium compounds improved the sensitivity and selectivity where H2O2 headed the oxidation of fluorescent cerium nanoparticles to non-fluorescent products which was ultimately quantitatively detected by fluorescence measurements with very low detection limit.

In addition to these fluorescent based biosensors working on the principal of fluorescence ON/OFF switching by nanomaterials, pronounced efforts have been made in this field with composite materials and modified nanomaterials. Moreover, quantitative measurement of fluorescence quenching phenomenon is also one of the effective tools of modern fluorescent biosensors. In this direction numerous materials have been used for biosensing applications such as, bovine serum albumin (BSA) protected Au nanoparticles were used to detect salicylaldehyde and Zinc ions [170]. Gold nanoparticles (AuNP) are the most widely studied material in the ever expanding world of nanomaterials. Chang et al. [171] used protected gold nanoparticles for hydrogen peroxide and cholesterol detection via fluorescent detection. Similarly, Huang et al. [172] used core-shell structured magnetic nanoparticles to detect cholesterol, use of nanoparticles improved the limit of detection and sensitivity of these fluorescence based biosensors. Overall, number of nanomaterials and composites have been used so far, as fluorescent and chemiluminescent probes mimicking the biomolecules as have been detailed in the Table 3.

Table 3 An overview on nanoparticle based fluorescent and chemiluminescent assays using enzyme mimics
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Chemiluminescence detection

Furthermore, a newer technique chemiluminescence (CL) is becoming important due to its higher sensitivity, rapidness, easier protocol and cost effective detection. Shi et al. used Co doped magnetic nanoparticles (CoFe2O4) for the first time for non-enzymatic chemiluminescence based sensitive detection of hydrogen peroxide and glucose [177]. The nanomaterial was more stable and sensitive as compared to the enzymes, which opened newer ways for the researchers and dragged several groups to work on similar lines.

Chen et al. employed cupric oxide nanoparticles with luminol-H2O2 system for the highly sensitive detection of hydrogen peroxide [100]. The CuO NPs enhanced the peroxidase mimicking of the nanocomposite 400 hundred times, due to its peroxidase like activity to decompose hydrogen peroxide into hydroxyl radicals. Similarly, another group utilized CoFe2O4 magnetic nanoparticles for luminol-H2O2 chemiluminescence system and found that the nanocomposite performed 10 times better catalytic activity, which was quenched on addition of trace levels of Ag (I). This phenomenon was further utilized for sensitive detection of Ag (I) from water samples with a detection limit of 0.15 ng mL−1.

Chemiluminescence has been utilized for determination of glucose using glucose oxidase which produces gluconic acid and hydrogen peroxide. The hydrogen peroxide can produce chemiluminescent signals on reacting with luminescent materials like luminol, rhodamine derivatives etc. which ultimately can be quantized in terms of concentration of glucose [179, 180].

Similarly, luminol assisted cupric oxide nanoparticles-based chemiluminescence sensor have been fabricated for cholesterol monitoring. Peroxidase-like activity of cupric oxide nanoparticles were used to oxidize luminol by hydrogen peroxide produced by cholesterol oxidase. Combination of these reactions resulted in chemiluminescence of luminol, the intensity of which was directly proportional to the cholesterol concentration, as have been demonstrated in the following Fig. 1 [76, 102].

Fig. 1
figure 1

Schematic representation of cupric oxide nanoparticles-based chemiluminescence sensor for cholesterol detection. Peroxidase-like activity of cupric oxide nanoparticles was used to oxidize luminol by hydrogen peroxide produced by cholesterol oxidase. Combination of these reactions resulted in chemiluminescence of luminol, the intensity of which was directly proportional to the cholesterol concentration [102]

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Applications of non-enzymatic assays

Biomedical assays

A brief overview regarding the use of these enzyme mimetic materials in fabricating biomedical assays for various analytes is given below:

Hydrogen peroxide and glucose detection

The detection of hydrogen peroxide is very much important in biological, clinical, environmental, and food industry. Over the past few years, several enzyme mimetics have been synthesized and subsequently utilized in developing sensor devices for hydrogen peroxide and glucose detection. AgVO3 nanobelts exhibit peroxidase like activity and was used for detecting H2O2 [28]. The production of CoFe2O4 nanotubes from electrospinning technique and subsequent functionalization with PdNPs enhances their peroxidase mimetic activity for sensitive detection of H2O2 [181]. A peroxidase mimetic layered double hydroxide (LDH) was prepared by electrostatic interaction between negatively charged carbon-dots and positively charged NiAl-LDH nanoplates [182]. A colorimetric sensor device for hydrogen peroxide was also fabricated by growing prussian blue (PB) on microporous MOFs having peroxidase like activity [183]. Magnetic nanoparticles of iron oxide also exhibit peroxidase-like activity. Other peroxidase like nanozymes including nanostructured layered double hydroxide [184], polyoxometalate/carboxyl-modified mesoporous polymers [185], and copper sulphide (concave structures) were used in sensing H2O2 and glucose [186]. FeS nano-sheets or nano needles furnish high surface area in comparison to nanoparticles [187, 188]. These nanostructures were used to fabricate electrochemical sensor for H2O2 based on their peroxidase like activity. More peroxidase mimics of ferrites were explored by doping Bi and Eu in FeO3 and Co, Zn, Mn in Fe2O4. Other peroxidase mimics were also explored such as PtPd porous nanorods [189], carbon nanodots (CDs), and PtPd nanodendrites on graphene nanosheets [190]. One dimensional PtPd porous nanorods exhibits lower detection limit of 8.6 nM for analyzing H2O2 released from living cells in comparison to all other nanostructures fabricated for detecting H2O2 [189]. PtNPs conjugated to apoferritin can mimic both catalase and peroxidase like activity depending on pH and temperature. Gold nanomaterials showed oxidase like properties and can serve as glucose oxidase (GOx) mimic and can convert adsorbed glucose to gluconic acid and H2O2. Enzyme mimic of AuNPs with other metals were also explored. Peroxidase like activity of Bi-AuNPs was used to develop fluorescent sensor for thrombin. Silver-based nanozymes (peroxidase-like) hybrid with other metals (Au, Pt, and Pd) were explored for monitoring ascorbic acid concentration [191]. In hybrid nanozymes, the polarization effects were induced in Fe3O4 by doping gold which enhances reduction property of Fe3O4 MNPs. Similarly, enhanced sensing capacity of Fe3O4 towards H2O2 was achieved by adding AgNPs [192]. Pronounced activity was also observed in FeTe nanorods in comparison to Fe3O4 MNPs due to its large surface area [193]. Peroxidase like activities were also observed in different forms of carbon materials such as carbon nanotubes (CNTs), graphene, carbon-nanodots, and carbon nanoparticles. Activities of CNTs are dependent on pH, temperature, H2O2, Fe contents and as well as its shape. The helical shape CNTs are more catalytically active than MWCNTs and expresses greater affinity to H2O2 than MNPs. Higher activity of graphene was achieved by combining it with hemin to detect DNA. However, peroxidase like activity of gold-modified graphene was inhibited due to adsorption of DNA which later on restored after releasing or cleavage of DNA resulting in developing a sensor for DNA. On the basis of this phenomenon, an insulin sensor was fabricated based on affinities of anti-insulin and insulin-aptamer complex [11]. A composite of V2O3 and mesoporous carbon also exhibits peroxidase like activity and utilized for colorimetric detection of H2O2 and glucose [194]. Other peroxidase mimics used for analyzing H2O2 and glucose are: VO2 nanobelts [105], MnSe-loaded graphene carbon nitride [195], graphene dots [196], and N-doped graphene QDs [197]. Hydrothermal synthesis of monocrystalline VO2 nanobelts possess an outstanding catalytic activity and exhibits very low detection limit of 0.65 μM towards glucose in comparison to all other nanomaterials used for glucose detection [105].

Cholesterol detection

Graphene shows higher affinity to targeted analytes than natural enzymes due to high surface area. Graphene-hemin hybrid nanosheets (nanozyme) express higher response to H2O2 than HRP. Graphene or graphene oxide based hybrid materials as nanozymes shows higher enzyme mimetic activities due to synergic effect. A synergic effect in peroxidase like properties was also observed in nanoribbons based on MWCNTS-rGO. This synergic effect is attributed to the higher electron-transfer rate in this heterostructure which is 15.9 folds higher than MWCNTs alone. This MWCNTs-rGO heterostructure was utilized to develop a selective colorimetric sensor system for cholesterol [198]. The incorporation of ZnO NPs on CNTs surface exhibit a synergistic peroxidase mimetic property for fabricating a sensor device for colorimetric detection of cholesterol with a detection limit of 0.2 nM and concentration range of 0.5–500 nM in comparison to all other sensors fabricated for cholesterol detection [199]. Hong et al. fabricated a chemiluminescent sensor for cholesterol based on peroxidase mimetic cupric oxide nanoparticles which shows LOD of 0.17 μM [102].

Other analytes

Different nanozymes based on metals, bi & tri-metals, metal oxides, metal-graphene hybrids have also been used in fabricating biomedical assays for detecting various analytes. Nanozymes based on AuNPs as peroxidase mimetics were utilized to detect alanine [200], and IgG [201], whereas, xanthine [202], choline and acetylcholine [203] was monitored by using nitrogen supported sulfur doped PdNPs and PtNPs in BSA scaffold. Moreover, bimetals (Pt/Au NPs) and rod-shaped nanostructures of trimetals (Au/Pt/Cu) were synthesized to analyze E-coli [204] and IgG [205], respectively. Enzyme mimetic hybrids of CuS and gold with graphene were used to detect respiratory syncytial virus [206], dopamine [207], DNA [208], and miRNA-21 [209]. Nanozymes that were used in developing biomedical assays for various analytes are summarized in the Table 4.

Table 4 An overview on nanoparticle based optical assays for biomedical analytes using enzyme mimics
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Environmental monitoring

In this part of review, environmental application of enzyme mimics would be slightly discussed and summarized in tabular form.

Chai et al. synthesized silver nanoparticles on silver phosphate microcubes (Ag@Ag3PO4MCs) microcubes by reacting [Ag (NH3)2]+ and Na2HPO4 under UV light for detecting Hg2+ ions selectively [216]. These microcubes exhibit oxidase-like activity and capable to oxidize 3,3′,5,5′-tetramethylbenzidine (TMB) and o-phenylenediamine (OPD) in slightly acidic media and dissolved oxygen. The oxidase-like properties of Ag@Ag3PO4 microcubes greatly enhanced by adding AgNPs (silver nanoparticles) in it which results in quicker oxidation of TMB and OPD. Mercury ions act as inhibitor for enzyme activity due to a very specific interaction with Ag and thus, on the basis of this inhibition property a colorimetric sensor for mercury ions were fabricated with LOD of 0.253 nM. Zhang et al. synthesized a very efficient hybrid material (Au/Fe3O4/GO) with enhanced nanozyme activity for ultrasensitive colorimetric detection of Hg2+ in aqueous solution with a detection limit as low as 0.15 nM [217]. The hybrid material was prepared by attaching gold and Fe3O4 nanoparticles on graphene oxide (GO). This ternary hybrid (Au/Fe3O4/GO) offers very lower detection limit in comparison to all other nanozymes used for detecting Hg2+. The graphene oxide in ternary hybrid furnishes large surface area for anchoring gold and Fe3O4 nanoparticles which reduces the chances for particle agglomeration and results in maximum enzymatic activity of gold for colorimetric detection of mercury ions. However, maximum detection ranges of 0–100 μM towards mercury ions was observed by using magnetic nanoparticles (MNPs) attached with target specific single-stranded DNA (ssDNA). Peroxidase mimetics of Pt were synthesized by using bovine serum albumin (BSA) as nucleation template for colorimetric detection of Hg2+ [218]. This BSA stabilized Pt nanozyme possess excellent peroxidase activity towards TMB and Hg2+ was selectively detected via Hg2+-Pt0 interaction by suppressing enzymatic activity of PtNPs. The lowest detection limit of 602 pM towards Pb2+ was achieved in case of colorimetric aptasensor triple-helix molecular switch (THMS) and enzyme mimetic AuNPs. However, maximum detection range for lead ions was observed in a nanozyme based on catechin-synthesized AuNPs.

Nanoceria has been synthesized to develop a colorimetric assay for the detection of toxic substance catechol with detection limit of 0.2 μM [219]. Peroxidase like activity of Cu2+ was utilized to develop a colorimetric sensor for detecting a very toxic pollutant glyphosate by using TMB as substrate [113]. Peroxidase mimetic magnetic nanoparticles were used for colorimetric detection of organophosphorus pesticides and nerve agents. Whereas, these magnetic nanoparticles were also utilized for selective detection of ethoprophos (pesticide) via chemiluminescence (CL) method [114, 115]. Highest detection range (0.1 nM - 100 μM) and lowest detection limit of 0.1 nM was observed towards ethoprophos while using enzyme mimetic magnetic nanoparticles. Different scientific approaches to tune nanozyme activities towards metal ions, pesticides, and environment pollutants are summarized in Table 5.

Table 5 An overview on nanoparticle based assays for pollutants using enzyme mimics
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Food analysis

In recent years, considerable attention has been focused on the food safety and food quality because of nutritional awareness, rapidly changing food choices and growing food consumption. As the food stuff and dietary products are directly associated with public health concern, therefore any contamination can cause a serious threat to living beings. Hence, the development of highly sensitive and rapid screening methods is of substantial interest to deal with food related health issues. Some interesting applications of non-enzymatic nanomaterial based sensors in the food analysis will be summarized in the following sections:

Pesticides

Extensive use of pesticides in agriculture, their high water solubility and high toxicity level in living beings demands the highly sensitive, rapid and accurate methods for the detection of these pollutants. A biomimetic sensor for the highly selective and sensitive determination of carbofuran pesticide was fabricated on carbon paste electrode immobilized with a composite of graphene oxide and hemin complex by Ademar et al. [230]. The efficiency of the sensor was scrutinized by employing cyclic voltammetry and square wave voltammetry. Fabricated assembly exhibited linearity between 5.0 × 10−6-9.5 × 10−5 mol. L−1 with a sensitivity of 1.1 × 105 μA.mol−1 and a limit of detection of 9 × 10−9 mol. L−1. A non-enzymatic simultaneous electrochemical detection of carbaryl (CBR) and carbofuran (CBF) pesticide was demonstrated by Wang et al. [231] based on the reduced graphene oxide film decorated with cobalt (II) oxide. CoO/rGO sensor exhibited a good response towards both pesticides in terms of two well-separated and well behaved DPV peaks for a mixed solution. The fabricated electrode showed a linear response over a wide range of concentrations i.e. 0.5–200 μM for CBR and 0.2–70 μM for CBF (S/N = 3) with a limit of detection of 7.5 and 4.2 μg. L−1 for CBR and CBF, respectively. This method was applied for the successful detection of the above mentioned pesticides in vegetable and fruit samples.

Antioxidants

Antioxidants in food are receiving much attention in recent years owing to their ability to improve health and prevent various diseases such as ageing, cardiac diseases and some neurodegenerative ailments e.g. Alzheimer’s and Parkinson’s disease. Keeping in view the physiological roles in maintaining the health, there is a growing interest to accurately access and evaluates the antioxidant capacity of dietary products. Vanillin, an antioxidant, was selectively determined by Lei’s group through cyclic voltammetry [232]. The electrochemical sensor for this purpose was fabricated by electrodepositing the Au-Pd alloy nanoparticles on reduced graphene oxide film. The hybrid film showed a linear electrocatalytic response towards vanillin ranges from 0.1–7 μM under optimal conditions with a detection limit of 20 nM and a sensitivity of 1.6 mA.mM−1.cm−2. The modified electrode was employed successfully for the determination of vanillin in vanilla biscuit, vanilla tea and vanilla beans. Lulia et al. [233] presented the voltammetry based quantitative determination of caffeic acid, a polyphenolic antioxidant, in three different tea types i.e. Turkish green, black and white teas by employing a disposable pencil graphite electrode. The sensor response was observed in terms of differential pulse voltammogram which exhibited a linear pattern with varying concentration of the analyte over a range 1 × 10−7-3 × 10−3 M with a detection limit of 8.83 × 10−8 M. The total phenolic contents for three tea types calculated from DPV were 35.81 (green), 34.59 (white) and 31.21 mg (black) caffeic acid equivalent/g tea. These values were found to be in accordance with those evaluated by Folin-Ciocalteu method, though well-known but more time consuming spectrophotometric methodology. A portable colorimetric sensor based on immobilized ceria nanoparticles was reported by Erica and group for detection of some antioxidants namely vanillic acid, ascorbic acid, quercetin, gallic acid, epigallocatechin gallate and caffeic acid [234]. The colorimetric response was found to be concentration dependent with a limit of detection over a wide concentration range i.e. from 20 to 400 μM depending upon the type of antioxidant involved. The fabricated sensor was applied successfully for evaluating the antioxidant contents in real samples i.e. medicinal mushrooms and teas. Owing to the multidimensional characteristics of ceria nanoparticles, this sensor was found to have many advantages over other systems which involve commercial oxidant kits, sensitive enzymes and conventional colorimetric dyes, in terms of fast response, cost effectiveness, portability, easy handling, high sensitivity and stability. The summaries of some non-enzymatic sensors used for various analytes associated with the food safety and quality assurance are described in Table 6.

Table 6 Non-enzymatic detection of analytes in food stuffs
Full size table

Conclusions and perspectives

Nano receptors based methodologies offer a novel and attractive paradigm in terms of new and augmented functionality that encompasses a wide variety of application in the analytical domain. This review provided a brief survey of the different types of nanomaterials, which are employed as a potential receptors element to replace natural enzyme in the field of biosensors, and have found widespread applications for biomedical, food and environmental analysis. These nano receptors are characterized with various advantages which included but not limited low cost, facile preparation, large scale synthesis, high stability and sustained catalytic activities. Nano receptor materials do not have real enzyme-like properties and it is not possible to regenerate nanomaterial surface in most of the cases for subsequent measurement, limiting their applications in amperometric biosensors or for repeated assays. Some attempts were made to regenerate the nano receptor surface for repeated cycles [249], but a decrease in the catalytic efficiency was observed after 8 cycles [250]. Moreover, controlling the reactivity of nanomaterials against certain interfering molecules is a very difficult task which may results in the generation of nonspecific signals, thus affecting the assays selectivity and specificity. The reactivity of nanomaterials is mainly related to the functional groups of the analytes, and closely related interfering molecules share a very similar structure to the analyte of interest and have possibility to react with the nanomaterials. This reactivity may result in generation of signals even in the absence of analyte and produce false positive results. To replace enzymes for biosensing applications, it is highly desirable to design selective and specific nanomaterials to overcome the matrix interferences. Moreover, future research may focus on the methods to regenerate the nano-surface to increase the reusability of the nano-sensors. Although, much progress has been seen in the use of these novel receptors to replace the natural enzyme, but there are few report on the integration of these materials in the design of affinity based sensors such as those based on the antibody or aptamer. For example, biomimetic nanomaterials have been employed to replace the common enzyme labels in the construction of immunosensors and aptasensors in some recent reports. We may envision the combination of the catalytic activity of nanomaterials and specificity of the antibody or aptamer molecules to construct novel designs of affinity based assays. Additionally, there are some points that need to be addressed in the design of non-enzymatic assays; 1) nanomaterial must be able to retain its catalytic activity under complex biological systems and under varying physiological conditions, as per requirement for most of the biological assays; 2) it would be of great interest to tune the nanomaterial’s surface in a controllable fashion for simultaneous measurements of various targets; 3) although, there is literature on the hybridization of various nanomaterials to design multicomponent systems, but most of the nanomaterials combinations are yet to explore in multicomponent systems for synergistic properties; 4) methods to overcome the nanomaterials aggregation under complex biological system should be focused and resolved.