Introduction

Graphene consists of a single layer of sp2 hybridized carbon atoms arranged in a honeycomb-like lattice [1]. Graphene has made its way into different fields of research including photocatalysis, batteries, electrochemical sensors and biosensors, gas sensors, and supercapacitors [2, 3]. Graphene is an interesting two-dimensional (2D) planar material which can be rolled into one-dimensional (1D) nanotubes, wrapped into zero-dimensional (0D) fullerenes. The sheets stacking result in the formation of three-dimensional (3D) graphite [1, 4]. Graphene displays amazing exceptional characteristics. It is the known thinnest material in the universe and the strongest ever measured material [5]. It is transparent with incredible flexibility [6]. Its optical transparency is ∼97.3% [7, 8]. Graphene mechanical strength is 200 times superior to steel [9]. Graphene has an exceptionally high theoretical surface area (2630 m2 g−1) which is substantially huge compared to graphite (∼10 m2 g−1) and two times larger than the carbon nanotubes (1315 m2 g−1) [10]. Graphene displayed the ultrahigh electron mobility 200,000 cm2 V−1 s−1 with charge density ∼2 × 1011 cm−2 [10,11,12]. Graphene possessed extraordinary thermal conductivity of 3000 WmK−1 [13, 14]. It is obvious that electrode materials perform a critical role in the fabrication of high-performance electrochemical sensor [15]. These unique and extraordinary physicochemical properties solidify the role of graphene in the field of material science and forthcoming technology.

Materials that are closely related to graphene are sometimes referred to as graphene. In some applications, both graphene oxide (GO) and reduced graphene oxide (rGO) also have been denoted as graphene. However, the pristine graphene has some obvious differences from GO and rGO. Graphene is a 2D material which only contains sp2 hybridized carbon atoms, whereas GO is a monolayer of graphite oxide. GO contains both sp2 and sp3 carbon atoms. The sp3 bonding in GO is around 40% [16]. It also has many oxygen-containing functional groups. The hydroxy and the epoxy groups are present on the basal plane and the sheet edges of the GO contain phenol, quinone, lactone, carbonyl, and carboxy groups. These covalently attached oxygen-containing functional groups generate extensive structural defects in the GO sheet compared to pristine graphene. These functional groups severely affect its mechanical and electrochemical properties. The poor electrochemical behavior put a limitation on the direct use of GO as an electrode material [17]. Electrochemical behavior of the GO is improved by reducing the GO with the help of chemical or electrochemical methods [18]. During reduction, certain oxygen-containing functional groups are lost which improve the electrochemical activity of the rGO. The carrier mobility of the chemically rGO is substantially lower compared to pristine graphene [16]. The conductivity of the rGO is improved up to an order of 4 compared to GO. The conductivity of the reduced GO is far behind (around a factor of 10–100) from pristine graphene [19]. Since the properties of the rGO are substantially different from graphene, it is not appropriate to refer rGO as graphene strictly [20]. Graphene quantum dots (GQDs) are small graphene pieces with nanometer size [21]. In GQDs the properties of the graphene and the quantum dots are merged. The different graphene and graphene-related materials are appropriately defined in Table 1.

Table 1 Definitions of graphene and graphene-related materials
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Graphene has proved a revolutionary outstanding electrode material due to its wide potential window, relatively inert behavior, and low cost. For electrochemical sensing, the graphene offered a wide potential window of 2.5 V in 0.1 M phosphate buffer (pH 7.0). Graphene has tremendous potential in the field of electrochemical sensing due to its exceptional electron transfer and the carrier mobility [28]. Moreover, it is lightweight, thermally and mechanically stable which made them an ideal material [29] for sensor fabrication. Modification of the electrode surfaces with graphene assists the electrode to achieve trace level quantification of the analytes with better reproducibility.

The intrinsic properties of graphene are severely compromised as the graphene nanosheets irreversibly agglomerate into a graphitic form. The restacking of the graphene nanosheets triggered from the basal plane of the graphene is due to the strong van der Waals forces and the π–interactions [30]. This agglomeration severely affects the intrinsic characteristics of graphene. As a result, the high surface area and exceptional conductivity of graphene is extensively compromised [31].

It is a challenge to preserve the intrinsic characteristics of graphene by inhibiting the restacking of graphene into graphitic form. The separation between the graphene sheets was tried to achieve by the incorporation of spacers [32] such as metals, metallic oxide, conductive polymers, carbon nanofiber (CNF) and carbon nanotubes (CNTs) [33,34,35,36,37,38]. The introduction of spacers somehow assists to separate the graphene layers. Researchers are constantly focusing on the new architectures of 2D graphene to achieve more refined results by overcoming the issue of restacking and agglomeration.

The 3D architecture of 2D graphene is an attempt to extend the successful utilization of the graphene. 3D porous materials with interconnected ordered structures are gaining technological importance in various applications. 3D architecture substantially improved the performance of the certain devices [39]. 3D graphene has several advantages to use it as an electrode material. The 3D graphene porous network for sensing applications provides a large surface area and facilitates the facile movement of electrolytes which allows the better contact of the analyte with the active sites of the sensing electrode. 3D graphene also acts as a scaffold for loading of other nonmaterial [40]. The 3D graphene provides a better opportunity for the loading of catalysts, enzymes, and nanomaterial to fabricate electrochemical sensors for selective and sensitive quantification of the analytes. 3D graphene continuous porous network prevents the aggregation of the metal nanoparticles or other functional materials which are loaded or embedded for certain sensing application. Different 3D materials including 3D macroporous Pt films, 3D Porous nickel nanostructures, 3D aperiodic hierarchical porous graphitic carbon, 3D CNT ensembles electrode, 3D CNF, nanodiamonds and 3D porous carbon were applied to the electrochemical applications [41,42,43,44,45,46,47]. This review is only focusing on 3D graphene-based electrochemical sensors.

The three-dimensional graphene design has a good future in many challenging applications such as catalyst support, stretchable conductors, environmental protection, biofuel cells, energy storage devices, gas sensors, and electrochemical sensors [48,49,50,51,52,53,54,55,56,57,58]. 3D graphene is a promising material for electrochemical sensing and proved an outstanding material for fabrication of electrochemical sensors [59]. The 3D porous network of graphene provides a platform to make different composites by using the porous network [60]. Expanding the 2D graphene into 3D architecture is a fascinating and further functionalization with other materials helps researchers to manipulate the material to any extent for desired results. The 3D graphene porous network is a promising electrode material due to light weight and excellent conductivity. There are only a few reviews on the 3D graphene porous network which mainly focus on synthesis and energy-related applications [8, 61,62,63]. There is no review written that specifically focuses on the application of 3D graphene porous networks for electrochemical sensing. There is a need to consolidate the work for future progress in the fabrication of 3D graphene electrodes.

Synthesis of three-dimensional graphene

Freestanding three-dimensional graphene or porous networks of graphene can be attained by various methods including template assembly and self-assembled graphene nanosheets [30]. The template strategy was commonly applied in several methods including chemical vapor deposition (CVD), solvothermal, sol-gel and hydrothermal synthesis to attain 3D porous graphene. Various templates such as Ni foam, Cu foam, polystyrene spheres, etc. were used for the fabrication of 3D graphene. The template-based synthesis of the 3D porous network generally consists of three steps. In the first step, the reaction precursors are attached to the template either by incorporation or impregnation. In the second step of the reaction, growth or nucleation is continued to form the solid species either in or on the template surface. In the last step, the template was removed by several ways to form a 3D porous structure [62]. A few methods based on a template or the templateless generation of 3D graphene is discussed.

Chemical vapor deposition

The chemical vapor deposition method is frequently applied for the synthesis of 3D graphene [64, 65]. The template-directed CVD method was used to synthesize the 3D foam consists of graphene macroscopic structure. In CVD method, the graphene is deposited on the 3D support by the decomposition of the various carbon-containing sources at high temperature. After deposition, the support is etched away chemically. Collapsing of graphene is prevented by depositing the thin layer of poly (methyl methacrylate) (PMMA) on the graphene film before etching process [66]. The 3D graphene network was generated on the nickel foam from the high-temperature decomposition of CH4. The 3D scaffold of Ni is etched away with the help of HCl to achieve a 3D continuous graphene network. Prior to etching, the graphene films were protected by poly (methyl methacrylate) (PMMA). The polymer was later on removed by the acetone treatment [66]. Xiehong Cao et al. [67] also used the CVD method based on nickel template foam for the generation of 3D graphene networks. Later on, the 3D graphene was coated with MWCNTs, Pt NPs, and MnO2 NPs to obtain various composites of 3D graphene. Similarly, 3D graphene was obtained to construct a monolithic free-standing electrochemical sensor [68]. The CVD method was used for in situ growth of cobalt oxide nanowires on the 3D graphene foam [31]. This approach facilitates the monolith fabrication of the 3D graphene network. In a continuous grown 3D graphene network, the charge carriers move more rapidly. CVD synthesized 3D graphene provides a high surface area on which various active materials can be anchored to impart specific characteristics for sensing applications. It also offers excellent conductive channels which allow the rapid electron transfer. CVD method also provides an opportunity to tune the pore size using metal-based nanotemplates which range from 50 nm to 1 μm [69]. The porous structure provides a platform for efficient contact between the electrode surface and the electrolyte [67]. The graphene network acquired from CVD displayed a good interconnection but poor mechanical stability. The mechanical issue arises due to the soft framework of pristine graphene. The large-scale production of 3D graphene is challenging due to the consumption of the template which leads to high production costs [70].

Hydrothermal method

The hydrothermal strategy was also applied to obtain a 3D graphene or rGO hydrogel. The rGO hydrogel is synthesized by sealing the GO into the autoclave reactor for several hours at high temperature. The PdCu nanoparticle (NP) decorated 3D graphene hydrogel was obtained from the aqueous dispersion of ethylene glycol, GO, glutamate and the salts of palladium(II) and copper(II). In a stainless-steel autoclave, the dispersion was sealed for several hours of heating to obtain a metallic nanoparticle decorated 3D rGO hydrogel [71]. In another work, self-assembled rGO hydrogel was obtained from GO using a one-step hydrothermal process (Fig. 1). The hydrogel was achieved by sealing GO into a Teflon-lined autoclave for heating at 180 °C for 1–12 h [72]. Similarly, the nitrogen-doped rGO hydrogel was obtained by gradually adding organic amine or ammonia into the GO prior to subjecting it to hydrothermal treatment [73]. Hydrothermal methods are simple and the 3D rGO composite with other nanomaterial is synthesized easily by mixing them with GO. Morphology can be controlled by those methods in which template are involved. In templateless methods, control of morphology is difficult to achieve.

Fig. 1
figure 1

a Photographs of a 2 mg/mL homogeneous GO aqueous dispersion before and after hydrothermal reduction at 180 °C for 12 h; b photographs of a strong SGH allowing easy handling and supporting weight; c–e SEM images with different magnifications of the SGH interior microstructures; f room temperature I–V curve of the SGH exhibiting Ohmic characteristic, inset shows the two-probe method for the conductivity measurements [72]. Copyright (2010) American Chemical Society

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Lithographically defined three-dimensional graphene structures

Lithography has emerged as a powerful tool for attaining 3D architectures [74]. 3D carbon can be achieved in lithography using photoresist structures. The thin nickel film was sputtered uniformly on the lithographically-defined conductive three-dimensional carbon networks. Further annealing of the Ni-coated carbon resulted in the fabrication of the multilayer graphene-coated 3D Ni electrode [75]. The multilayer graphene hollow structure can form by removing the Ni through acid etching (Fig. 2). Overall three steps are required to convert the predefined 3D pyrolyzed photoresist films into 3D porous graphene. In the first step, the Ni was sputtered on the 3D amorphous carbon structure; while the second step consisted of annealing. In the last step, the Ni was removed by acid etching to give a multilayer graphene hollow structure (Fig. 3) [76]. One of the major advantages to using the porous network is the huge available surface area and availability of the interior surface for the electrochemical reaction. Sometimes, the nanopores high density and the wide size-distribution of the pores may lead to the overlapping diffusion regions which may limit the mass transport to the interior region of the porous network. The benefit of the high surface area does not remain much effective [75]. The highly ordered fabrication of the mesoporous material might be a possible solution. Lithography provided more controlled fabrication of 3D interconnected porous electrode. The controlled morphology provided several pathways for the analyte to diffuse and the electrochemical reaction takes place throughout the porous electrode surface. The nanofabrication through conventional lithography is a complex, expensive and time-consuming process [77].

Fig. 2
figure 2

SEM images of the lithographically generated porous carbon (a), porous carbon coated with nickel before thermal annealing (b), and 3D graphene structure after annealing at 750 C° and etching of Ni layer in a 2 M H2SO4 solution (c, d) [76]. Copyright (2012) American Chemical Society

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Fig. 3
figure 3

Schematic drawings illustrating the steps and mechanism for chemical conversion of amorphous porous carbon to 3D graphene: a porous carbon, b conformal Ni coating, c diffusion of carbon into Ni top surface during thermal annealing, and d hollow 3D graphene after Ni etching [76]. Copyright (2012) American Chemical Society

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Support assisted and chemically deposited three-dimensional graphene

Reduction of the GO is very common by using different reducing agents. GO is reduced with the help of a reducing agent on a 3D support to obtain a 3D network of rGO. Fei Liu et al. [78] fabricated a 3D rGO architecture using polydimethylsiloxane (PDMS) micropillars. The GO was adsorbed on the PDMS micropillar and reduced chemically with hydrazine vapors. Freeze drying is another method to obtain a 3D network. There are two main steps involved in synthesizing 3D rGO by the freeze-drying method. In the first step, the GO was reduced by using a reducing agent such as hydrazine hydrate. In the second step, the rGO was freeze-dried to obtain a 3D network [59]. Similarly, a 3D-AuNP/rGO composite was also synthesized by reducing the GO and the HAuCl4·4H2O using polyethylene glycol as a reducing agent. After reducing, it was freeze-dried [79]. Sacrificial templates were also used for the synthesis of 3D graphene. Polystyrene is the commonly used sacrificial template [80]. The graphene spheres were generated by mixing the positively charged polystyrene spheres into the negatively charged GO solution. Due to the electrostatic force of attraction; the negatively charged GO sheets wrapped around the polystyrene spheres to neutralize the charge. A high number of GO sheets are wrapped due to the π–interactions. The spherical shell wrapped GO sheets were reduced with hydrazine to achieve the PS@graphene core-shell structure. The polystyrene template was removed by heat treatment to obtain the graphene hollow spheres [81]. For the electrochemical sensor application, 3D rGO nanosheets were also fabricated using a polystyrene spheres template. After the reduction of GO, the template was removed by the toluene sonication [82].

Three-dimensional graphene-polyaniline hybrid hollow spheres (3D rGO-PANI HS) were also fabricated using polystyrene as a template. GO was reduced in the presence of polystyrene sulfonate. The sulfonated group imparts the negative charge to the rGO and provides a stable dispersion of negatively charged rGO. The positive charge PANI dispersion was also produced. A rGO-PANI multilayer encapsulated PS sphere was fabricated using a layer by layer assembly method by the interaction of the negative charge rGO and positively charged PANI. The PS core was removed with the help of tetrahydrofuran to get 3D rGO-PANI hollow spheres (Fig. 4) [83]. The fabrication of the 3D graphene by the chemical deposition looks simple and cost-effective. The 3D network is obtained from the GO. GO synthesis and handling are convenient. Due to the hydrophilic nature of GO, it is easily dispersed in the aqueous medium [84]. GO is reduced using various reducing agents. The reduction of GO can introduce more defects into the rGO. Furthermore, the reduction of the GO not able to restore the pristine graphene network and some oxygen functional group still retained by the surface which substantially affects the performance of the material. Morphology of the 3D rGO is random and poorly controlled by this method.

Fig. 4
figure 4

The illustration of the fabrication procedure of the graphene–PANI hollow spheres (RGO-PANI HS). In the first step, the PANI wrapped PS sphere was formed by mixing negatively charged PS sphered with positively charged PANI. In the second step, the negatively charged RGO sheets self-assembled on PANI@PS by dipping PANI@PS into RGO dispersion. The first and second steps are multiple times repeated to produce (RGO–PANI)n@PS. In the final step, the PS core was removed by dipping in tetrahydrofuran (THF) to generate RGO-PANI HS [83]. Copyright (2015) Elsevier Ltd.

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Direct electrochemical methods

Three-dimensional rGO can be fabricated on the surface of an electrode directly either by applying a constant [85] or by sweeping a range of potential. This method can be utilized to synthesize 3D rGO composites with metals or metallic oxide. The fabrication process for the composites simply consists of two steps. In the first step, the bulk of the GO is directly reduced electrochemically and in second step, the noble metal nanoparticles, metal oxide nanoparticles or conductive polymer are also electrochemically deposited on the surface due to the conductive network of the 3D rGO [86]. 3D rGO and the metallic nanoparticles composite are synthesized by one-step electrochemical reduction. Fan Shi et al. [87] fabricated a three-dimensional rGO–Au nanocomposite on the electrode surface by one step electrochemical co-deposition. The 3D rGO–Au nanocomposite was formed on the surface by sweeping the potential in the range of −1.5 to 0 V using cyclic voltammetry. Similarly, the constant potential (−1.2 V) also has a tendency to provide a 3D interpenetrating porous graphene which is also a one-step process [88]. In another application, the constant potential of −1.3 V was applied to form a 3D rGO film on the CILE surface [89]. The SEM images of the three-dimensionally grown electrochemical rGO can be seen in Fig. 5. The electrochemical methods are the simplest one compared to other methods. 3D rGO films are generated on the surface of the solid electrode simply by applying constant negative or sweeping potential. In this method no special chemical agent is required and the reduction takes place by the exchange of electrons between GO and the electrodes These methods help to avoid the use of dangerous reducing agents and their byproducts [20]. The surface is self-assembled into the 3D network during electrochemical reduction of GO. The major disadvantage of the electrochemical method is the uncontrolled morphology. There is no control on the pore size and the porous network of the 3D rGO.

Fig. 5
figure 5

SEM images the surface of ErGO modified electrode with different magnifications, respectively [88]. Copyright (2013) Elsevier Ltd.

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Other methods

Apart from these methods, several other methods were also implemented for the fabrication of 3D graphene. Xuebin Wang et al. [70] introduced a new method for the bulk production of 3D graphene. 3D self-supported graphene was grown by using a sugar-blowing technique and it was named strutted graphene (SG). It consisted of homogenous and continuous connected networks of the graphitic membrane with μm width graphitic struts spatially supporting the network (Fig. 6). This unique way of synthesis produced 3D graphene with a high mechanical strength, electrical conductivity, elasticity and a huge surface area. This method also provides an opportunity for large-scale production of 3D graphene. The 3D graphene or rGO network can also designed by the 3D printing method [90].

Fig. 6
figure 6

The 3D bubble-like network of SG. a Scanning electron microscope (SEM) image and an optical photo of a 70-mg SG piece obtained at a heating rate of 4 °C min−1. The inset is a reconstructed topology corresponding to the region marked in red for the two connected decahedron–dodecahedron bubbles faced by eight pentagons and two quadrangles, and by eight pentagons, three quadrangles and one heptagon, respectively. b SEM image of the flat backside of the SG product with a regular arrangement of connected cells identified by artificial quilt colours. c, d 2D and 3D optical photos of SG networks. e Photos of original compressed and recovered states of a SG specimen. (f) XRD profile of SG. Scale bars, 200 μm (in a–d) and 1 cm (in e) [70]. Copyright (2013) Springer Nature

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The aforementioned discussion of the synthesis revealed that the 3D graphene architecture can be fabricated in numerous ways and with a variety of materials. The most common approach adopted for the synthesis of a 3D architecture consists of either template or templateless synthesis. Both of these approaches have their own merits and demerits. The template method provides the easiest route to form the 3D network of the graphene. Pore size and the morphology are difficult to control in templateless synthesis while template provides an opportunity to control the porosity of the 3D graphene. The pore size was generated in the range of 600 nm to 4 μm using the PS sphere as a template to obtain 3D porous carbon scaffolds. PS spheres are simply replaced by heat treatment [91]. The 3D metal templates such as Ni-template is used to obtain porous network with the pore size in the range of 50 nm to 1 μm [69]. The most successful method for control of porosity and the morphology of the 3D graphene is a template-directed CVD method. The template with fine pores and range of porosity are required to tune the pore size of the 3D graphene porous network. In CVD methods the template removal during the etching process is a challenge to acquire a defect-free 3D network. Apart from CVD, the templates are used in other synthesis methods including hydrothermal, freeze drying, and support assisted chemical deposition. In these methods, the morphology control is poor compared to the CVD method. Among them, the simplest method is the electrochemically generated 3D network of rGO. In this method, either a constant or sweeping potential is required to produce the rGO. In the electrochemical method, the morphology of the 3D rGO cannot be controlled due to the absence of a template. The freeze drying method is also applied for the templateless synthesis of 3D rGO. GO is the commonly used precursor to attain 3D architecture. GO is reduced by these methods either chemically or electrochemically to improve the electrochemical behaviors. During reduction process, only graphene extraordinary properties partially restored due to retaining of some oxygen functional group. Through these methodologies, it is not an easy job to get a defect-free 3D graphene network. Although, new methods are continuously being introduced, including 3D printing through which 3D free-standing graphene can be achieved with much more control compared with other methods. More serious efforts are required to produce 3D defect-free graphene architectures which are mechanically stable and display the intrinsic characteristics of pristine graphene. Table 2 displays a critical comparison of various methods.

Table 2 Critical comparison of the various methods applied for 3D graphene synthesis
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Key features of 3D graphene composites and its application in electrochemical sensing

The 3D hierarchical architectures of graphene include some remarkable physicochemical properties which are very different from both bulk and individual building blocks [55, 94]. Three-dimensional graphene has demonstrated certain advantages which make it an attractive material for the construction of ultrasensitive chemical sensors. Some of the key features of the 3D graphene are described below [31, 67, 78, 88, 95,96,97,98,99,100,101].

  1. 1)

    3D graphene can prevent the agglomeration of planar graphene sheets by overcoming the π–interactions.

  2. 2)

    3D graphene architecture may also help to restore the intrinsic characteristics of the graphene.

  3. 3)

    3D graphene can be prepared from free of defects and the intersheet junctions.

  4. 4)

    The 3D interconnected network of the graphene provides the multiple electron paths which facilitate the rapid and sensitive detection of the analyte.

  5. 5)

    The interconnected open porosity of the 3D graphene facilitates the kinetic diffusion and mass transfer of macromolecules.

  6. 6)

    The 3D graphene has low density and high porosity.

  7. 7)

    The 3D monolithic and macroporous graphene foam can act as a free-standing 3D electrode.

  8. 8)

    The small molecules can easily adsorb into 3D graphene hydrogel.

  9. 9)

    The porous network of graphene also facilitates the stable loading of the catalyst and provides a huge surface area for enzyme immobilization.

  10. 10)

    The scaffold of the 3D porous graphene network provides an ideal opportunity to fabricate monolithic composite and hybrid electrodes.

Application of 3D graphene and its composite based electrode for electrochemical sensing are further discussed in detail.

Three-dimensional porous graphene-based electrodes

Three-dimensional graphene provides a promising platform for the sensing of various electrochemical analytes. The specific surface area of the 2D electrodes is limited and attempts are being made to construct 3-dimensional electrodes. The 3D graphene network provides multiplexed conductive pathways and offers rapid charge transfer. These factors contribute to enhance the sensitivity of the electrochemical sensor. Highly conductive and monolithic 3D graphene foam was synthesized by CVD and was applied for sensing of dopamine [68]. The sensing of dopamine is challenging due to its close electrooxidation peak with uric acid [102]. The 3D graphene electrodes demonstrate the tendency to distinguish between the dopamine and the uric acid peaks. The sensor displayed a wide linear range of 25 nM to 25 μM. The excellent sensitivity, selectivity and wide linear range of the sensor are attributed to high charge transfer, availability of the high active surface area and π–interactions of the dopamine with graphene. [68]. Similarly, electrochemically synthesized 3D ErGO on AuE displays high conductive channels and strong electrocatalytic behavior for the sensing of dopamine compared to compact 2D ErGO. In compact 2D ErGO provided a limited excess to electrolyte containing dopamine and fewer graphene edges are exposed for the electrocatalysis which is responsible for lower sensitivity [103]. Bo Yu et al. [82] also fabricated a 3D rGO sensor for the sensing of dopamine. A facile way was adopted to attain 3D graphene using a template-assisted self-assembly method in which polystyrene was used as a sacrificial template. The GO and the polystyrene self-assembling were generated by π–interactions. The rGO after removal of the template displayed a 3D network. Electrochemical activity and the sensitivity of the 3D rGO were better when compared to rGO and the poly-(vinylpyrrolidone) (PVP)-stabilized rGO. The enhanced sensitivity of the 3D rGO compared the rGO is due to the open pore structure of the 3D rGO. The sensor displayed sensitivity of 244.17 μA μM−1 cm−2 at an amperometric potential of +0.3 V which is better [82] than 3D graphene electrodes (0.6196 μA μM−1 cm−2). However, the 3D graphene electrode was operated at a lower potential of +0.177 V for the amperometric sensing of dopamine [68]. Chitosan assisted electrochemically reduced 3D GO modified sensors improved the stability and the activity of the immobilized enzyme glucose oxidase (GOD). The fabricated biosensor demonstrated good electrocatalytic activity towards the dissolved oxygen. Direct electrochemistry of immobilized glucose oxidase occurred on the modified electrode which leads to the conversion of GOD(FAD) to GOD(FADH2) due to fast electron transfer between the graphene-modified electrode and the enzyme. It electrocatalyzed the dissolved oxygen reduction which leads to the regeneration of GOD(FAD) from GOD(FADH2). In this way, the electrocatalytic reaction can be repeated again and again. The reduction current of the air saturated solution decreased with glucose spiking due to the interaction of glucose with GOD(FAD). This helps to attain a wide linear range of the glucose. The fabricated biosensor was highly selective for glucose in the presence of potentially interfering species [88]. In another work, the 3D rGO was generated on the surface of the CILE using the direct electrochemical method to immobilize the hemoglobin for biosensor fabrication. Hemoglobin displayed a direct electrochemistry on the surface of the 3D rGO/CILE. The biosensor catalyzed the reduction of trichloroacetic acid in the absence of electron mediators [89]. In another work, the graphene aerogel@octadecylamine-functionalized carbon quantum dots modified GCE displayed a wide linear response of 0.001–10 μM for acetaminophen determination [104]. The 3D hollow graphene balls were also used for the sensing of levodopa in the presence of uric acid [105].

The fast charge transfer of the 3D graphene composite also assists the sensor for the real-time application. For example, the real-time application of the nitric oxide is challenging due to trace level of the endogenous NO and its reaction with oxygen is critically fast with very short half-life. A sensor with fast response time is only suitable for real-time sensing [106]. The 3D rGO/IL nanocomposite was prepared by mixing 3D rGO and the 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid to fabricate a carbon paste electrode for sensitive sensing of NO. During amperometric measurements, the steady state current was achieved after spiking of the NO in less than 4 s which made it a good candidate for the in-situ or in-vivo applications [107]. Table 3 demonstrated the figure of merits of 3D porous graphene-based electrodes.

Table 3 Figure of merits of three-dimensional porous graphene-based electrodes. PB stands for phosphate buffer.
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3D graphene metals and metals oxide composite modified electrodes

The composites of 3D graphene with metals nanoparticles offer a platform for the sensitive and selective sensing of various targeted species. The metal nanoparticles act as a catalyst during electrochemical reactions [108, 109]. The metallic nanoparticle modified sensor performance is affected due to the agglomeration and dissolution of the electrocatalyst during the catalytic reaction. In this regard, either graphene or other carbon materials are used as a support [110] to improve the catalyst durability, electroactive surface area, and surface sensitivity. The cooperative interaction of graphene and metal nanoparticles promotes the electron transfer. However, the catalyst performance, electrolyte diffusion, and the electroactive surface area are compromised due to the agglomeration of graphene by π–interactions [111]. 3D graphene architecture is the best alternative to fix the issue of 2D graphene agglomeration. A immunosensor was fabricated for the detection of carcinoembryonic antigens (CEA) by using 3D-AuNP/rGO. The immunosnesor was achieved by immobilizing the primary antibodies (Ab1) into a 3D porous network of rGO. The CEA was attached to the sensor by an antigen-antibody relation. The electrochemical signal was generated with the assist of thionine and horseradish peroxidase (Fig. 7). The immunosensor demonstrated a very low limit of detection of 0.35 pg mL−1 for CEA [79]. The combination of 3D graphene hydrogel and Au NPs was also applied to the simultaneous detection of AA, DA, and UA [112]. The bimetallic nanoparticles of PdCu were encapsulated and dispersed within the 3D interconnected network of rGO. The 3D rGO provided an opportunity for the uniform dispersion of the PdCu NPs on both sides of the rGO sheets. The synergistic effect of 3D rGO and the bimetallic nanoparticles significantly improved the electrocatalytic oxidation of glucose and its selectivity over potential interfering species [71]. Similarly, the 3D porous graphene foam was used for the incorporation of bimetallic nanoparticles of platinum and ruthenium which act as a nanocatalyst for hydrogen peroxide determination. The ultra-hollow structure of graphene foam with its large number of active sites provides the opportunity for the uniform dispersion of bimetallic nanoparticles. Sometimes amperometric biosensors are inaccurate for finding out the concentration of H2O2 due to their low sensitivity. The porous 3D graphene provides rapid mass transport of the reactant to the nanocatalyst and enhances the sensitivity for H2O2. The sensitivity of the bimetallic nanoparticle sensor with 2D graphene was 0.7954 which were substantially improved to 1.0231 μA·μM−1·cm−2 by replacing graphene with 3D graphene foam. The detection limit was also improved significantly from 0.355 to 0.04 μM [113]. The 3D graphene as a free-standing electrode demonstrated a high catalytic activity for methanol oxidation compared to the 3D scaffold of carbon fiber [114].

Fig. 7
figure 7

Fabrication process of HRP-Ab2/TH/NPS nanomaterials and measurement protocol of the electrochemical immunosensor. In the first step, 3D-AuNPs/GN deposited on the GCE then incubated for 35 min and washed to remove unspecific adsorption. After that, it was incubated in 1.0 wt% BSA for 30 min at room temperature. In next step, Ab1/3D-AuNPs/GN/GCE immunosensor, was incubated with CEA standard antigen and unbounded CEA molecule remove by extensive washing. A sandwich immunocomplex was constructed by dropping HRP-Ab2/TH/NPS onto the modified electrode. DPV was used for sensing of CEA [79]. Copyright (2013) Elsevier B.V.

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The combination of metallic oxide and 3D graphene was also applied to certain analytes in which metallic oxide acts as a catalyst while 3D graphene assists the rapid charge transfer during the electrochemical reaction. The cobalt oxide (Co3O4) nanowires on the 3D graphene were used for the fabrication of a monolithic free-standing electrode which is applied to the enzymeless sensing of glucose. The Co3O4 is proficient enough to catalyze the glucose oxidation and enable the electrode to sense glucose without other mediators or enzymes. The 3D graphene/Co3O4 composite due to the superior mechanical strength of the graphene, it can be used as a free-standing electrode with better sensitivity for glucose sensing [31]. In another work, in situ ZnO nanorods were fabricated on CVD grown 3D graphene. The high conductivity of the 3D graphene and the electroactive properties of the ZnO facilitate the rapid charge transfer which is required for dopamine sensing [60]. The figure of merits for the 3D graphene metals and metals oxide composite modified electrodes are described in Table 4.

Table 4 Figure of merits of 3D graphene metals and metals oxide composite modified electrodes. PB stands for phosphate buffer
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3D graphene and organic polymer composite modified electrodes

The combination of polymers with graphene is generally used to improve the selectivity and the sensitivity of the sensor [115, 116]. Mostly, conductive polymers are used for the fabrication of electrochemical sensors [117]. Conductive polymers are a class of polymers which contain numerous sp2 hybridized carbon atoms and their large resonating structure allows the delocalized transport of charge carriers [118]. The hybrid of 3D graphene with polymer composite was also used for the sensing of various analytes with improved selectivity. For example, the oligonucleotides display a great tendency for specific interaction with metal ions. The DNA makes a stable and strong complex with mercury ions through thymine [119]. The thymine–Hg2+–thymine complexes were used for the sensing of mercury ions [120]. A 3D-rGO@PANI nanocomposite was used to fabricate a biosensor for the selective determination of mercury ions by immobilizing the DNA. The 3D rGO substantially enhanced the electrochemical performance and the specific surface area of the nanocomposite for the immobilization of DNA. The biosensor was selective towards the mercury ions as it binds with T-rich DNA by making T-Hg2+-T coordination which increased the biosensor impedance. The biosensor was very sensitive for Hg2+ with a limit of detection 0.035 nM [59]. Similarly, a biosensor was obtained by immobilizing the DNA on the nanocomposite of 3D rGO and plasma-polymerized propargylamine (3D-rGO@PpPG) for the sensitive and selective determination of Hg2+ [120].

Fei Liu et al. [78] introduced a 3D rGO micropillar electrochemical biosensor consist of tyrosinase enzyme for the sensitive detection of phenol. In the microfluidic channels, 3D PDMS micropillars were patterned and the surface was modified with 3-aminopropyltriethoxysilane. GO adsorbed on the PDMS through electrostatic force of attraction and reduced with hydrazine to attain a 3D rGO conductive architecture (Fig. 8). Tyrosinase enzymes catalyzed the introduced phenol into catechol and catechol was further oxidized into o-quinone which electrochemically reduced back to catechol. The tyrosinase enzyme-mediated biosensor was specific for phenol which reduces the chance of interference.

Fig. 8
figure 8

a, b Low- and c high-magnification SEM images of the 3D graphene micropillars. d The PDMS micropillars uncovered with graphene are shown bright due to the charging effect. e Digital image of the graphene micropillar integrated electrochemical sensor device for phenol detection [78]. Copyright (2013) Elsevier B.V.

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In some cases, the polymer was introduced onto the surface of 3D graphene to act as a linker for further attachment of functional groups. Hydrophilicity is one of the factors for the loading and preservation of the bioactivity of the biomolecules during bioanalysis. The 3D graphene foam is highly hydrophobic. The polymer such as polydopamine (pDA) performs two functions; first, it imparts the hydrophilic character to 3D graphene and, second, act as a linker for the attachment of other groups. The dopamine can polymerize in-situ in the alkaline medium on the 3D graphene to act as a linker and render the hydrophilic character to the surface [121]. The polydopamine efficiently adhere to the surface of the graphene via π–stacking interaction [64]. It is established that ambiphilic pDA have strong bond formation tendency with various hydrophilic and hydrophobic surfaces using their ortho-dihydroxy-phenyl functional groups [121]. The adhered pDA provides ample reactive handles that have the capabilities to conjugate with thiol- or amine-containing molecules via Michael addition or Schiff base reactions [64]. Jiyang Liu et al. [121] used the polydopamine linker to functionalize the concanavalin A monolayer on the 3D graphene. The horseradish peroxidase (HRP)-labeled antibody (anti-CEA) was immobilized further to develop a 3D immunosensor which have the capability to sense carcinoembryonic antigen with wide linear range and very low limit of detection (Fig. 9) [121]. In another example, polydopamine linker on the 3D graphene surface act as a linker for the functionalization of the thionine molecules which has the capability to efficiently mediate the enzymeless reduction of H2O2. Thionine is oxidized to facilitate the reduction of H2O2 to H2O and reduced back to thionine via electron transfer on the electrode surface. It was applied for real-time sensing of H2O2 released from cancer cells in response to a pro-inflammatory stimulant [64]. Table 5 demonstrates the figure of merits of 3D graphene and organic polymer composite modified electrodes.

Fig. 9
figure 9

Schematic illustration of the fabrication and CEA detection process of the immunosensor. 3D-G/pDA formed by the in-situ polymerization of dopamine in alkaline medium. The concanavalin A (Con A) was covalently immobilized by linking through polydopamine on the electrode surface. After that, the horseradish peroxidase (HRP)-labeled antibody (anti-CEA) was efficiently immobilized to demonstrate the recognition interface [121]. Copyright (2014) Elsevier B.V.

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Table 5 Figure of merits of 3D graphene and organic polymer composite modified electrodes. PB: phosphate buffer
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CNT-decorated 3D graphene-based electrodes

The synergistic effect of CNT and graphene helps to attain the robust and the flexible electrodes [116]. The SWCNTs can act as the spacer, binder and conductive additive to the composites [122]. The CNTs decoration of 3D graphene further improves the electroactivity of the surface. A ferrocene-branched chitosan (Fc-CS), single-walled carbon nanotubes (SWCNTs) and glucose oxidase (GOD) modified Fc-CS/SWCNTs/GOD/3DG electrode was used for the electrochemical sensing of glucose. The immobilization of the ferrocene grafted chitosan on the 3D graphene was stable and retained its original activity. The SWCNTs in Fc-CS matrix act as nanowires to improve the conductivity and electron transfer of the biocomposite film. The biosensor was fabricated by immobilizing the biocomposite which contained a glucose oxidase (GOD) using a one-step electrodeposition on the 3D graphene foam [123]. The chitosan solubility is pH dependent and at a higher pH (> 6.3) it becomes insoluble [124, 125]. A 3D graphene electrode was dipped into a solution containing the GOD, SWCNTs, and Fc-CS. At an applied potential of −1.5 V, the pH was increased near the electrode surface due to the reduction of H+ to H2. The chitosan became insoluble as the local pH exceeded 6.3. As a result, the GOD enzyme and SWCNTs incorporated into the CS Fc-CS hydrogel were electrodeposited onto the 3D graphene electrode. This provides the easiest way to fabricate a reagentless enzymatic glucose sensor by bio-modification of monolithic 3D-G foam using a one-step CS electrodeposition technique [123]. As discussed, the 3D graphene porous network can also use as a template for the loading of the various nanomaterial to fabricate the graphene-based composite electrodes for enhancing the sensing capability of the sensor. Xiehong Cao et al. [67] also synthesized 3D graphene architecture in which various nanomaterial successfully deposited. The combination of the entangled MWCNTs and the 3D graphene porous network displayed a hierarchical structure. At the edges, numerous protruded MWCNT tips were observed. These extended tips serve as electron transfer conduction channels and enhance the activity towards the electrochemical reaction. The functionalization of Pt NPs on the 3DGN/MWCNTs significantly improved the electrochemical activity of the composite towards H2O2. Overall these composites reduce the overpotential compared to 3D graphene. Table 6 describes the figure of merits of functionalized 3D graphene and MWCNTs-decorated 3D graphene-based electrodes.

Table 6 Figure of merits of functionalized 3D graphene and MWCNTs-decorated 3D graphene-based electrodes
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Miscellaneous

Apart from the above combination, the 3D graphene can also combine with other materials to enhance the electroactivity of the sensors. The 3D graphene or rGO is doped with heteroatoms to improve the electrochemical behavior of the graphene [126, 127]. Generally, those heteroatoms are used for doping in graphene which has a close resemblance in electronegativity and atomic structure, to carbon [128]. The 3D phosphorous doped rGO was synthesized by a hydrothermal and annealing treatment. The 3D porous network of the rGO and the phosphorous doping synergistic effect enable the sensor to sense trace level H2O2 released from the living cells (HeLa) [127]. Similarly, the 3D ammonia doped porous rGO/CuO modified sensor was applied to the non-enzymatic sensing of glucose. The composite was achieved on the gold electrode surface by directly applying the DC voltage in an ammonia doped porous rGO and Cu(ClO4)2 suspension using electrophoretic deposition. The film thickness was controlled by the deposition time [126]. In another work, 3D nitrogen-doped graphene was fabricated which provided a favorable environment for the immobilization of the single-stranded DNA probe [129]. In some cases, the graphene was used with other biomolecules or nanomaterial to construct 3D nanocomposites. The ZnO nanorods were used for the fabrication of a 3D rGO/Ni hybrid electrode for a non-enzymatic glucose sensor [130]. In another work, the 3D composite was fabricated on the surface of the gold electrode using graphene and DNA. The 3D structure was built using a layer by layer assembly to construct a sensitive and selective sensor for dopamine sensing [94]. Figure of merits of the miscellaneous 3D graphene-based electrodes are described in Table 7.

Table 7 Figure of merits of miscellaneous 3D graphene-based electrodes
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Future perspectives, challenges, and conclusion

After the day of isolation, a significant progress in graphene material is made and with the passage of time, the new approaches are being introduced to effectively use graphene in different applications. Apart from the huge progress in graphene, still it is difficult to prepare graphene in bulk which is fully displaying its intrinsic properties. The key challenges are the agglomeration of graphene sheets and the defects that are generated during the synthesis. 3D graphene architecture is the newly introduced morphology of the planar graphene which is continuously getting attention from a couple of years in the different field. It has full potential to prove a promising electrode material for the fabrication of sensitive and selective electrochemical sensors for trace level quantification of the various analytes. 3D graphene not only possesses the properties of the intrinsic graphene but also provides some additional benefits. It prevents the agglomeration of the graphene sheets and allows the free movement of the electrolyte into the porous graphene network. The analyte has more contact with the 3D graphene compared to 2D planar graphene. It also offers facile and more stable loading of catalyst and enzyme due to porous network, for the fabrication of electrochemical sensor as well as a biosensor. Although in numerous sensing applications, the 3D graphene was fabricated on the surface of the conventional electrode, nevertheless, the free-standing 3D graphene has an obvious advantage over 2D planar graphene network, as it can directly use as an electrode.

The 3D graphene porous network and the huge surface area enable the stable composites with other active materials to achieve the desired results. The various composites of 3D graphene are introduced with other nanomaterials including metallic/metallic oxide nanoparticles, conductive polymers, and MWCNTs. The 3D graphene provides better dispersion of the nanoparticles and helps to improve their electrocatalytic behavior. The 3D graphene also provides an opportunity to functionalize the surface of the graphene with different biomolecules to enhance the sensitivity and the selectivity of the sensors towards specifically targeted analytes.

Actually, all the unique properties of the 3D graphene depend on the efficient preparation of the material. The 3D G/rGO was obtained by several methods, including electrochemical methods [86], template-assisted methods [66, 131] and a self-assembled strategy [132, 133]. Mostly graphene is referred to rGO and the precursor is the GO. The GO is widely used due to its more feasible synthesis route and it can easily disperse into the aqueous medium. It can be reduced very easily by chemical or electrochemical methods [134, 135]. The rGO electrochemical properties are significantly different from graphene due to the partial removal of the oxygen-containing functional groups. The sp2 planar structure of the graphene cannot fully restore due to the retaining of the oxygen-containing functional groups. Synthesis of graphene from GO by the synthetic route is extremely difficult [136]. Another challenge to obtain a 3D graphene network with controlled porosity. In templateless methods, the porosity is difficult to control and 3D graphene network grown with random morphology. The pore size of 3D porous network somehow controlled using the template which provides an opportunity to fabricate the porous material according to the guest molecule size. 3D porous carbon scaffolds with few nanometers to several micrometers pore size were fabricated using PS spheres as a template for bacterial cells confinement [91]. A graphene skeleton with a large pore size of 100–200 μm is also reported [137] A commonly used method for obtaining 3D graphene with controlled morphology is the template-assisted CVD method [138]. The template in the CVD allowed better tuning of the porosity and the morphology of the 3D graphene. 3D nanoporous graphene with pore size of 50 nm to 1 μm synthesized using Ni-based nanotemplates [69]. However, the removal of the template without damaging the 3D graphene network is a challenge and most defects are generated during its removal which affects the sensor performance. The 3D graphene can also obtain by a sugar blowing technique with random geometry and the pore size [139]. It is obvious that the performance of the 3D graphene-based sensors depends on the efficient synthesis of the defect-free graphene network. The high level of geometry and shape control of the 3D graphene was attempted by 3D printing strategies [140]. Apart from all these methods, more exploration is required to introduce new facile methods for the synthesis of 3D free-standing graphene networks without any defects. 3D graphene, along with other widespread applications, has also been explored in electrochemical sensing. However, it is in its infancy stage regarding the fabrication of electrochemical sensors and very few applications have been reported including the sensing of biomolecules, organic and inorganic pollutants. 3D graphene material has great potential in the field of sensing for fabrication of highly selective and sensitive electrochemical sensors. Due to its unique porosity and 3D network, it provides the improved potential for loading the desired biomolecules for biosensor fabrication. The large biomolecules can also easily access to the tuned pore size of the 3D graphene porous network. This material can bring more useful results by the functionalization of the 3D graphene surface for specific analytes. The functionalization assists the 3D graphene to preconcentrate the targeted analyte into the porous network. This result into a substantial improvement in sensor selectivity, as well as sensitivity, compared to its 2D planar counterpart.

There is no doubt that 3D graphene is an extraordinary electrode material but some serious challenges are associated with the fabrication of 3D graphene porous network. The unique and extraordinary characteristics of the 3D graphene are only depends upon the successful construction of the material. The porosity of the material until now is not precisely controlled and pore size is distributed in the range of nm to μm. Moreover, during preparation, many defects are generated in the architecture which severely affects their performance in the electrochemical sensing. The mechanical stability of the 3D network of the graphene still needs to be fixed to make them valuable sensing tool. There is also need to put more focus on the determination of defects during construction the material and find outs the methods to eliminate them. This will help to improve the mechanical stability and electrochemical behavior of the 3D graphene electrodes. More theoretical and experimental exploration of 3D graphene with other nanocomposite is required to obtain the optimum synergistic effect for selective and sensitive sensing of the analytes. 3D graphene has a promising future in the field of electrochemical sensing for the fabrication of low cost and sensitive sensors.