- Email: [email protected]
Phosphorene and black phosphorus for sensing and biosensing
Accepted Manuscript Phosphorene and Black Phosphorus for Sensing and Biosensing Martin Pumera PII:
S0165-9936(17)30103-6
DOI:
10.1016/j.trac.2017.05.002
Reference:
TRAC 14921
To appear in:
Trends in Analytical Chemistry
Received Date: 21 March 2017 Revised Date:
12 May 2017
Accepted Date: 13 May 2017
Please cite this article as: M. Pumera, Phosphorene and Black Phosphorus for Sensing and Biosensing, Trends in Analytical Chemistry (2017), doi: 10.1016/j.trac.2017.05.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Phosphorene and Black Phosphorus for Sensing and Biosensing
RI PT
Martin Pumera*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences
SC
Nanyang Technological University Singapore 637371 (Singapore); [email protected]
Abstract
Introduction
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Two dimensional (2D) materials exhibit highly useful materials properties. Graphene, single sheet transition metals dichalcogenides found plethora applications in various fields, including analytical chemistry. Layered black phosphorus and its single sheet variation (phosphorene) became popular material very recently due to its monoelemental composition, biocompatibility, electrochemical properties, tunable bandgap and resulting optical properties. Here we describe progress which was made towards analytical applications of black phosphorus and its single layer counterpart, phosphorene.
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Two-dimensional materials have become very popular in the field of analytical chemistry. With their unique chemistry and physics, they unlocked the doors of new conceptual applications in analytical chemistry. This started with graphene1 and related materials (such as graphane2, fluorographene3, graphene oxide4, graphene quantum dots5,6, and others), which were applied for myriads of analytical applications.
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However, many other 2-dimensional materials are of very high interest and importance for analytical chemistry. Layered and 2D transition metal dichalcogenides of general formula MX2, where M is a transition metal, such as Mo, Se, V, Ti, Ta, or Pt and X is a chalcogen, such as S, Se, or Te (a typical example of this category of compounds is MoS2)7 demonstrated applicability for gas-8 and solution-based9 sensing, as fluorescence quenchers10, biomolecule labels11, or electrode surfaces.12 In similar way, there was interest in layered MXenes.13 In the past three years, there has been a dramatic increase in the interest in an elemental layered material, black phosphorus.14 Black phosphorus, even though first prepared in 1914 by Bridgman15, remained a curiosity in materials science until very recently, when its materials properties attracted a huge amount of attention in the materials science community. Black phosphorus has a layered structure, where two layers of phosphorus atoms are interconnected in a “washboard”
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structure and create one sheet (Figure 1) which, if projected from the axis perpendicular to the basal plane, shows a hexagonal structure. Black phosphorus is non-toxic16 (in contrast with white phosphorus) and it exhibits a band gap (in contrast with graphene), which increases with a decrease in the number of layers.17 A single layer of black phosphorus is called phosphorene. Black phosphorus, in its multilayer form or as a single layer has found applications in batteries18, polymer composites19, and field effect transistors20, to name just a few. It may not be surprising that black phosphorus made its way into the analytical sciences. Here, we wish to add to the previous literature regarding trends in the use of 2D materials in analytical chemistry, published on graphene21,22 and transition metal dichalcogenides23, and to discuss the emerging applications of black phosphorus in the field of analytical chemistry.
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Figure 1. Structure of black single layer of black phosphorus (phosphorene). Top and side view of the sheet. Reproduced from [36] with permission.
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Humidity and gas sensing
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Black phosphorus and phosphorene show high sensitivity to humidity, turning the top layer of BP into hydrated phosphorus oxides. This process is oxygen- and UV light-dependent and accelerates with increasing band gap and thus decreased number of BP layers.24 However, it appears that the surface of BP is passivated as even nanometer-sized quantum dots are stable in aqueous dispersions for weeks.25 The sensitivity to water (and vapor in general) can be used to advantage when the material is used as a gas or vapor sensor. Theoretical studies were carried out to investigate how gas adsorption influences the band gap and thus sensing properties on a variety of gases, such as CO, CO2, NH3, NO, and NO2. It was shown that the highest differences should be obtained from nitrogen-containing molecules. NH3 was expected to reduce the current in a field effect transistor (FET) while NO was expected to increase the current in the device.26
Figure 2. Gas sensing with black phosphorus. (a) Schematic set-up of black phosphorus between two gold electrodes. (b) Experimental (dotted line) and simulated (full line) data for the impedance module of layered black phosphorus in the absence (air) and in the presence of 1140 ppm methanol vapor. Inset: Equivalent electrical circuit for modeling the impedance module data. (c) Impedance phase spectra of the black phosphorus device in the presence of
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different methanol vapor concentrations. (d) Impedance phase at 1 kHz as a function of the methanol vapor concentration. Reprinted from [27] with permission.
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The first gas sensor based on black phosphorus used impedimetric transduction. It was shown that the phase shift of BP in the presence of methanol shows maxima at a frequency different than that for all other typical vapors (water, DMF, ethanol, chloroform, toluene, and isopropanol) and thus allowed selective detection of methanol (Figure 2). The sensitivity of the phase shift in impedance measurements is very high, allowing measurement of methanol concentrations down to 28 ppm, which is significantly below approved limits of detection.27 The different sensitivity and selectivity of the sensor can be ascribed to different adsorption energies of the organic compounds on BP, in similar way they show them of fluorographene.28 It was shown that black phosphorus nanosheets are much more sensitive sensors of gases than are graphene and MoS2 sheets, especially for water vapor and NO2.29 A whole plethora of papers on black phosphorus humidity and gas sensors has recently been published. For example, black phosphorus nanosheet humidity sensors were fabricated based on exfoliated black phosphorus using liquid exfoliation in N-methyl-2-pyrrolidone30 or electrochemical exfoliation.31 Both devices were based on the changing resistivity of black phosphorus (BP) sheets during exposure to relative humidity.30,31 An ammonia (NH3) sensor was tested based on resistance changes in BP nanosheets (increasing the resistance of BP upon adsorption of NH3, in accordance with the theoretical study discussed above26) exfoliated from BP using liquid-phase exfoliation with N-cyclohexyl-2-pyrrolidone, with detection limits down to low ppm levels.32 Kitchen blender-induced exfoliation and downsizing of black phosphorus sheets to nanoparticles, which were in turn employed as humidity sensors, were again based on the changes in resistivity of BP nanoparticles.33 It should be noted that the resistivity changes most likely originate from changing contact resistance between the sheets during changing humidity.
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Ultrasonication exfoliation is also a popular method for downsizing black phosphorus to fewlayer flakes and to nanoparticles.25 This method was also used to create black phosphorus “paper” via filtration of exfoliated sheets (exfoliation carried out in DMF or DMSO) over the membrane and using it as a resistive humidity sensor.34
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Deeper understanding of gas sensing was conveyed by Chen et al, who studied the dependency of NO2 sensing on the sheet thickness of black phosphorus. The authors found that the sensitivity increases up to a thickness of 4.8 nm, and decreases with thinner BP layers. The authors suggested a model where two contradictory effects play a role, first an increased band gap for thinner sheets and by the effective thickness on gas adsorption for thicker sheets (Figure 3).35
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Figure 3. NO2 gas sensing depends on the black phosphorus thickness. (a) Dynamic response curves of relative conductance change versus time for NO2 concentrations ranging from 20 to 1,000 ppb (balanced in dry air). A drain-source voltage of 0.6 V was applied to the device. The dashed line demonstrates the ‘on/off’ of NO2 gas. (b) Thickness-dependent multi-cycle responses of the PNS sensor to 500 ppb NO2. (c) Calibration curves of the 4.8-nm-thick BP sensors to NO2 gas. (d) Trend line for the thickness-dependent multi-cycle responses in b. (e) Dynamic sensing response curve of the 4.8-nm PNS to various gases, including 10,000 ppb CO, 100,000 ppb H2, 10,000 ppb H2S and 100 ppb NO2. The sensor shows a much higher response to NO2 compared with other gases. ∆G/G0 is relative conductance change. Reproduced from [35] with permission.
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Sensing and biosensing in solutions
Black phosphorus has distinct properties. It is conductive in its bulk form and it can serve as an electrode surface.36 It is non-toxic and, therefore, is an ideal material for biosensing.16 It is electrocatalytic for some reactions and due to its band gap its nanoparticles exhibit fluorescence. All of these properties can be used for sensing. We first focus on the electrochemistry. Black phosphorus shows a wide electrochemical cathodic window, as it is not highly electrocatalytic for hydrogen evolution. However, its anodic window is somewhat limited due to the electrochemical oxidation of black phosphorus itself, ~0.6 V (vs. Ag/AgCl) at neutral pH.36 Black phosphorus shows significantly anisotropic electrochemical properties, with much faster heterogeneous electron transfer at the edge sites when compared to the basal plane37, in
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fashion similar to that of graphite38 and MoS2.39 This was used for an example for the detection of ascorbic acid and dopamine at the edge plane sites of a black phosphorus monocrystal electrode.37 Cyclic voltammetry and electrochemical impedance measurements were employed to detect hydrogen peroxide at the black phosphorus nanosheet surface.40 A mechanically exfoliated sheet of black phosphorus was placed between two electrodes and covered with ionophore in order to fabricate a sensitive ion sensor towards pollutants such as As3+, Pb2+, Cd2+, or Hg2+ (Figure 4).41 This sensor was able to detect Pb2+ down to ppb levels.
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Figure 4. Solution based sensing on black phosphorus. (A) Black phosphorus is highly anisotropic and electrochemical detection of (a) ascorbic acid and (b) dopamine exhibit very different response on the edge sites and basal plane of BP. (B) Preparation and characterization of few
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layer black phosphorus ion-sensor. (C) Response of the BP with ionophore layer to (a) As3+ and (b) Cd2+. Reprinted with permission from [37] (A) and [41] (B, C).
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An electrochemical myoglobin sensor was developed based on black phosphorus sheet functionalization with poly-L-lysine and a myoglobin-specific aptamer.42 This complex was immobilized at the screen-printed electrode and detection of myoglobin based on Fe2+/Fe3+ chemistry was performed (Figure 5, A). The ability to catalyze a hydrogen evolution reaction (albeit not as strongly as MoS2 or other transition metal dichalcogenides43-45) enables the use of black phosphorus nanoparticles as labels for immunoassays. The magnetic beads were functionalized with anti-rabbit IgG and rabbit IgG was functionalized with electrochemically exfoliated black phosphorus nanoparticles. After conjugation, the system was placed on a screen-printed electrode surface and exposed to strong acid. The detached black phosphorus nanolabels impacted the surface of the electrode and from the frequency of the impacts of BP NPs catalyzing the hydrogen evolution reaction indicated the concentration of rabbit IgG was devised (Figure 5, B).46 A field effect transistor (FET) fabricated from few-layer black phosphorus was prepared in order to detect IgG via anti-IgG linked to Au NPs placed on top of the BP sheet (Figure 5, C).47
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Figure 5. (A) Electrochemical myoglobin sensors. (B) The use of the black phosphorus nanosheets as labels for magneto immunoassay. (C) FET transisors based biosensors based on black phosphorus. Reprinted with permission from [42] (A), [46] (B) and [47] (C).
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The fluorescence of black phosphorus quantum dots can be utilized in a biosensing process. It was shown that an ssDNA probe labeled with a fluorescence quencher adsorbed at BP quantum dots desorbs upon successful hybridization with complementary ssDNA, yielding larger amounts of free BP quantum dots, which results in stronger fluorescence (Figure 6).48
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Figure 6. Schematic representation of the DNA detection strategy based on BPNPs as fluorophores.
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We have shown that a new 2D material, phosphorene, and its 3D counterpart, black phosphorus, have quickly found their way into the realm of analytical chemistry. Black phosphorus was used to fabricate gas and vapor sensors, with analytes ranging from water vapor to NO2 gas. In solution-based sensing techniques, analytes ranging from hydrogen peroxide to immunomolecules and DNA were detected in various electrochemical, electrical, and fluorescence transducing modes. While phosphorene has experienced rapid adoption in the field of analytical chemistry, much remains to be done in discovering the unique properties of this exciting material. This include chemical functionalization of the BP surface to prevent degradation of the material and to tailor its chemical properties, the understanding the influence of crystallinity of BP, the number of layers of BP and the sheet size to various analytical parameter. Further doping of black phosphorus with other atoms shall tailor the properties of the black phosphorus. The analytical chemistry will benefit from rise of understanding of physical chemistry and synthetic chemistry of black phosphorus (phosphorene) as well of antimonene, arsenene and bismuthene.49
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References
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1 M. Pumera, A. Ambrosi, A. Bonanni, E. L. K. Chng, H. L. Poh, Trends Anal. Chem. 2010, 29, 954. 2 S. M. Tan, Z. Sofer, M. Pumera, Electroanal 2013, 25, 703 3 V. Urbanová, F. Karlický, A. Matěj, F. Šembera, Z. Janoušek, J. A. Perman, V. Ranc, K. Čépe, J. Michl, M. Otyepka, R. Zbořil, Nanoscale, 2016,8, 12134. 4 N. Morimoto, T. Kubo, Y. Nishina, Sci. Rep. 2016, 6, 21715. 5 A. H. Loo, Z. Sofer, D. Bousa, P. Ulbrich, A. Bonanni, M. Pumera, ACS Appl. Mater. Interfaces 2016, 8, 1951. 6 L.-L. Li, J. Ji, R. Fei, C.-Z. Wang, Q. Lu, J.-R. Zhang, L.-P. Jiang, J.-J. Zhu, Adv. Funct. Mater. 2012, 22, 2971. 7 S. Li, S. Wang, D.-M. Tang, W. Zhao, H. Xu, L. Chu, Y. Bando, D. Golberg, G. Eda, Appl. Mater. Today 2015, 1, 60. 8 C. C. Mayorga-Martinez, A. Ambrosi, A. Y. S. Eng, Z. Sofer, M. Pumera, Adv. Funct. Mater. 2015, 25, 5611. 9 C. C. Mayorga-Martinez, B. Khezri, A. Y. S. Eng, Z. Sofer, P. Ulbrich, M. Pumera, Adv. Funct. Mater. 2016, 26, 4094 10 A. H. Loo, A. Bonanni, M. Pumera, Analyst, 2016, 141, 4654 11 R. J. Toh, C. C. Mayorga-Martinez, Z. Sofer, M. Pumera, Adv. Funct. Mater. 2017, 27, 1604923. 12 M. Pumera, Z. Sofer, A. Ambrosi, J. Mater. Chem. 2014, 2, 8981. 13 J. Zhu, A. Chroneos, J. Eppinger, U. Schwingenschlögl, Appl. Mater. Today, 2016, 5, 19. 14 R. Gusmao, Z. Sofer, M. Pumera, Angew Chem. 2017, in press. 15 P. W. Bridgman, J. Am. Chem. Soc. 1914, 36, 1344. 16 N. M. Latiff, W. Z. Teo, Z. Sofer, A. C. Fisher, M. Pumera, Chem Eur J 2015, 21, 19991. 17 M. Batmunkh, M. Bat-Erdene, J. G. Shapter, Adv. Mater. 2016, 28, 8586. 18 M. C. Stan, J. Von Zamory, S. Passerini, T. Nilges, M. Winter, J. Mater. Chem. A 2013, 1, 5293. 19 D. Li, A. E. del Rio Castillo, H. Jussila, G. Ye, Z. Ren, J. Bai, X. Chen, H. Lipsanen, Z. Sun, F. Bonaccorso, Appl. Mater. Today 2016, 4, 17. 20 H. Liu, A. T. Neal, Z. Zhu, Z. Luo, X. Xu, D. Tomanek, P. D. Ye, ACS Nano 2014, 8, 4033–4041. 21 A. Bonanni, A. H. Loo, M. Pumera, Trends Anal. Chem. 2012, 37, 12. 22 A. Martín, A. Escarpa, Trends. Anal. Chem. 2014, 56, 13. 23 M. Pumera, A. H. Loo, Trends Anal. Chem. 2014, 61, 49. 24 A. Favron, E. Gaufrès, F. Fossard, A.-L. Phaneuf-L’Heureux, N. Y.-W. Tang, P. L. Lévesque, A. Loiseau, R. Leonelli, S. Francoeur, R. Martel, Nat. Mater. 2015, 14, 826. 25 Z. Sofer, D. Bouša, J. Luxa, V. Mazanek, M. Pumera, Chem. Commun. 2016, 52, 1563. 26 L. Kou, T. Frauenheim, C. Chen, J. Phys. Chem. Lett. 2014, 5, 2675. 27 C. C. Mayorga-Martinez, Z. Sofer, M. Pumera, Angew. Chem. Int. Ed. 2015, 54, 14317. 28 E. Otyepková, P. Lazar, K. Čépe, O. Tomanec, M. Otyepka, Appl. Mater. Today 2016, 5, 142. 29 S.-Y. Cho, Y. Lee, H.-J. Koh, H. Jung, J.-S. Kim, H.-W. Yoo, J. Kim, H.-T. Jung, Adv. Mater. 2016, 28, 7020. 30 D. J. Late, Microporous Mesoporous Mater. 2016, 225, 494. 31 M. B. Erande, M. S. Pawar, D. J. Late, ACS Appl. Mater. Interf. 2016, 8, 11548.
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Layered black phosphorus and phosphorene show biocompatibility. Phosphorene has interesting electrochemical properties, optical properties and tunable bandgap.
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The analytical applications of black phosphorus are overviewed.
S0165-9936(17)30103-6
DOI:
10.1016/j.trac.2017.05.002
Reference:
TRAC 14921
To appear in:
Trends in Analytical Chemistry
Received Date: 21 March 2017 Revised Date:
12 May 2017
Accepted Date: 13 May 2017
Please cite this article as: M. Pumera, Phosphorene and Black Phosphorus for Sensing and Biosensing, Trends in Analytical Chemistry (2017), doi: 10.1016/j.trac.2017.05.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Phosphorene and Black Phosphorus for Sensing and Biosensing
RI PT
Martin Pumera*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences
SC
Nanyang Technological University Singapore 637371 (Singapore); [email protected]
Abstract
Introduction
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Two dimensional (2D) materials exhibit highly useful materials properties. Graphene, single sheet transition metals dichalcogenides found plethora applications in various fields, including analytical chemistry. Layered black phosphorus and its single sheet variation (phosphorene) became popular material very recently due to its monoelemental composition, biocompatibility, electrochemical properties, tunable bandgap and resulting optical properties. Here we describe progress which was made towards analytical applications of black phosphorus and its single layer counterpart, phosphorene.
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Two-dimensional materials have become very popular in the field of analytical chemistry. With their unique chemistry and physics, they unlocked the doors of new conceptual applications in analytical chemistry. This started with graphene1 and related materials (such as graphane2, fluorographene3, graphene oxide4, graphene quantum dots5,6, and others), which were applied for myriads of analytical applications.
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However, many other 2-dimensional materials are of very high interest and importance for analytical chemistry. Layered and 2D transition metal dichalcogenides of general formula MX2, where M is a transition metal, such as Mo, Se, V, Ti, Ta, or Pt and X is a chalcogen, such as S, Se, or Te (a typical example of this category of compounds is MoS2)7 demonstrated applicability for gas-8 and solution-based9 sensing, as fluorescence quenchers10, biomolecule labels11, or electrode surfaces.12 In similar way, there was interest in layered MXenes.13 In the past three years, there has been a dramatic increase in the interest in an elemental layered material, black phosphorus.14 Black phosphorus, even though first prepared in 1914 by Bridgman15, remained a curiosity in materials science until very recently, when its materials properties attracted a huge amount of attention in the materials science community. Black phosphorus has a layered structure, where two layers of phosphorus atoms are interconnected in a “washboard”
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structure and create one sheet (Figure 1) which, if projected from the axis perpendicular to the basal plane, shows a hexagonal structure. Black phosphorus is non-toxic16 (in contrast with white phosphorus) and it exhibits a band gap (in contrast with graphene), which increases with a decrease in the number of layers.17 A single layer of black phosphorus is called phosphorene. Black phosphorus, in its multilayer form or as a single layer has found applications in batteries18, polymer composites19, and field effect transistors20, to name just a few. It may not be surprising that black phosphorus made its way into the analytical sciences. Here, we wish to add to the previous literature regarding trends in the use of 2D materials in analytical chemistry, published on graphene21,22 and transition metal dichalcogenides23, and to discuss the emerging applications of black phosphorus in the field of analytical chemistry.
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Figure 1. Structure of black single layer of black phosphorus (phosphorene). Top and side view of the sheet. Reproduced from [36] with permission.
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Humidity and gas sensing
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Black phosphorus and phosphorene show high sensitivity to humidity, turning the top layer of BP into hydrated phosphorus oxides. This process is oxygen- and UV light-dependent and accelerates with increasing band gap and thus decreased number of BP layers.24 However, it appears that the surface of BP is passivated as even nanometer-sized quantum dots are stable in aqueous dispersions for weeks.25 The sensitivity to water (and vapor in general) can be used to advantage when the material is used as a gas or vapor sensor. Theoretical studies were carried out to investigate how gas adsorption influences the band gap and thus sensing properties on a variety of gases, such as CO, CO2, NH3, NO, and NO2. It was shown that the highest differences should be obtained from nitrogen-containing molecules. NH3 was expected to reduce the current in a field effect transistor (FET) while NO was expected to increase the current in the device.26
Figure 2. Gas sensing with black phosphorus. (a) Schematic set-up of black phosphorus between two gold electrodes. (b) Experimental (dotted line) and simulated (full line) data for the impedance module of layered black phosphorus in the absence (air) and in the presence of 1140 ppm methanol vapor. Inset: Equivalent electrical circuit for modeling the impedance module data. (c) Impedance phase spectra of the black phosphorus device in the presence of
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different methanol vapor concentrations. (d) Impedance phase at 1 kHz as a function of the methanol vapor concentration. Reprinted from [27] with permission.
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The first gas sensor based on black phosphorus used impedimetric transduction. It was shown that the phase shift of BP in the presence of methanol shows maxima at a frequency different than that for all other typical vapors (water, DMF, ethanol, chloroform, toluene, and isopropanol) and thus allowed selective detection of methanol (Figure 2). The sensitivity of the phase shift in impedance measurements is very high, allowing measurement of methanol concentrations down to 28 ppm, which is significantly below approved limits of detection.27 The different sensitivity and selectivity of the sensor can be ascribed to different adsorption energies of the organic compounds on BP, in similar way they show them of fluorographene.28 It was shown that black phosphorus nanosheets are much more sensitive sensors of gases than are graphene and MoS2 sheets, especially for water vapor and NO2.29 A whole plethora of papers on black phosphorus humidity and gas sensors has recently been published. For example, black phosphorus nanosheet humidity sensors were fabricated based on exfoliated black phosphorus using liquid exfoliation in N-methyl-2-pyrrolidone30 or electrochemical exfoliation.31 Both devices were based on the changing resistivity of black phosphorus (BP) sheets during exposure to relative humidity.30,31 An ammonia (NH3) sensor was tested based on resistance changes in BP nanosheets (increasing the resistance of BP upon adsorption of NH3, in accordance with the theoretical study discussed above26) exfoliated from BP using liquid-phase exfoliation with N-cyclohexyl-2-pyrrolidone, with detection limits down to low ppm levels.32 Kitchen blender-induced exfoliation and downsizing of black phosphorus sheets to nanoparticles, which were in turn employed as humidity sensors, were again based on the changes in resistivity of BP nanoparticles.33 It should be noted that the resistivity changes most likely originate from changing contact resistance between the sheets during changing humidity.
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Ultrasonication exfoliation is also a popular method for downsizing black phosphorus to fewlayer flakes and to nanoparticles.25 This method was also used to create black phosphorus “paper” via filtration of exfoliated sheets (exfoliation carried out in DMF or DMSO) over the membrane and using it as a resistive humidity sensor.34
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Deeper understanding of gas sensing was conveyed by Chen et al, who studied the dependency of NO2 sensing on the sheet thickness of black phosphorus. The authors found that the sensitivity increases up to a thickness of 4.8 nm, and decreases with thinner BP layers. The authors suggested a model where two contradictory effects play a role, first an increased band gap for thinner sheets and by the effective thickness on gas adsorption for thicker sheets (Figure 3).35
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Figure 3. NO2 gas sensing depends on the black phosphorus thickness. (a) Dynamic response curves of relative conductance change versus time for NO2 concentrations ranging from 20 to 1,000 ppb (balanced in dry air). A drain-source voltage of 0.6 V was applied to the device. The dashed line demonstrates the ‘on/off’ of NO2 gas. (b) Thickness-dependent multi-cycle responses of the PNS sensor to 500 ppb NO2. (c) Calibration curves of the 4.8-nm-thick BP sensors to NO2 gas. (d) Trend line for the thickness-dependent multi-cycle responses in b. (e) Dynamic sensing response curve of the 4.8-nm PNS to various gases, including 10,000 ppb CO, 100,000 ppb H2, 10,000 ppb H2S and 100 ppb NO2. The sensor shows a much higher response to NO2 compared with other gases. ∆G/G0 is relative conductance change. Reproduced from [35] with permission.
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Black phosphorus has distinct properties. It is conductive in its bulk form and it can serve as an electrode surface.36 It is non-toxic and, therefore, is an ideal material for biosensing.16 It is electrocatalytic for some reactions and due to its band gap its nanoparticles exhibit fluorescence. All of these properties can be used for sensing. We first focus on the electrochemistry. Black phosphorus shows a wide electrochemical cathodic window, as it is not highly electrocatalytic for hydrogen evolution. However, its anodic window is somewhat limited due to the electrochemical oxidation of black phosphorus itself, ~0.6 V (vs. Ag/AgCl) at neutral pH.36 Black phosphorus shows significantly anisotropic electrochemical properties, with much faster heterogeneous electron transfer at the edge sites when compared to the basal plane37, in
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fashion similar to that of graphite38 and MoS2.39 This was used for an example for the detection of ascorbic acid and dopamine at the edge plane sites of a black phosphorus monocrystal electrode.37 Cyclic voltammetry and electrochemical impedance measurements were employed to detect hydrogen peroxide at the black phosphorus nanosheet surface.40 A mechanically exfoliated sheet of black phosphorus was placed between two electrodes and covered with ionophore in order to fabricate a sensitive ion sensor towards pollutants such as As3+, Pb2+, Cd2+, or Hg2+ (Figure 4).41 This sensor was able to detect Pb2+ down to ppb levels.
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Figure 4. Solution based sensing on black phosphorus. (A) Black phosphorus is highly anisotropic and electrochemical detection of (a) ascorbic acid and (b) dopamine exhibit very different response on the edge sites and basal plane of BP. (B) Preparation and characterization of few
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layer black phosphorus ion-sensor. (C) Response of the BP with ionophore layer to (a) As3+ and (b) Cd2+. Reprinted with permission from [37] (A) and [41] (B, C).
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An electrochemical myoglobin sensor was developed based on black phosphorus sheet functionalization with poly-L-lysine and a myoglobin-specific aptamer.42 This complex was immobilized at the screen-printed electrode and detection of myoglobin based on Fe2+/Fe3+ chemistry was performed (Figure 5, A). The ability to catalyze a hydrogen evolution reaction (albeit not as strongly as MoS2 or other transition metal dichalcogenides43-45) enables the use of black phosphorus nanoparticles as labels for immunoassays. The magnetic beads were functionalized with anti-rabbit IgG and rabbit IgG was functionalized with electrochemically exfoliated black phosphorus nanoparticles. After conjugation, the system was placed on a screen-printed electrode surface and exposed to strong acid. The detached black phosphorus nanolabels impacted the surface of the electrode and from the frequency of the impacts of BP NPs catalyzing the hydrogen evolution reaction indicated the concentration of rabbit IgG was devised (Figure 5, B).46 A field effect transistor (FET) fabricated from few-layer black phosphorus was prepared in order to detect IgG via anti-IgG linked to Au NPs placed on top of the BP sheet (Figure 5, C).47
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Figure 5. (A) Electrochemical myoglobin sensors. (B) The use of the black phosphorus nanosheets as labels for magneto immunoassay. (C) FET transisors based biosensors based on black phosphorus. Reprinted with permission from [42] (A), [46] (B) and [47] (C).
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The fluorescence of black phosphorus quantum dots can be utilized in a biosensing process. It was shown that an ssDNA probe labeled with a fluorescence quencher adsorbed at BP quantum dots desorbs upon successful hybridization with complementary ssDNA, yielding larger amounts of free BP quantum dots, which results in stronger fluorescence (Figure 6).48
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Figure 6. Schematic representation of the DNA detection strategy based on BPNPs as fluorophores.
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We have shown that a new 2D material, phosphorene, and its 3D counterpart, black phosphorus, have quickly found their way into the realm of analytical chemistry. Black phosphorus was used to fabricate gas and vapor sensors, with analytes ranging from water vapor to NO2 gas. In solution-based sensing techniques, analytes ranging from hydrogen peroxide to immunomolecules and DNA were detected in various electrochemical, electrical, and fluorescence transducing modes. While phosphorene has experienced rapid adoption in the field of analytical chemistry, much remains to be done in discovering the unique properties of this exciting material. This include chemical functionalization of the BP surface to prevent degradation of the material and to tailor its chemical properties, the understanding the influence of crystallinity of BP, the number of layers of BP and the sheet size to various analytical parameter. Further doping of black phosphorus with other atoms shall tailor the properties of the black phosphorus. The analytical chemistry will benefit from rise of understanding of physical chemistry and synthetic chemistry of black phosphorus (phosphorene) as well of antimonene, arsenene and bismuthene.49
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Layered black phosphorus and phosphorene show biocompatibility. Phosphorene has interesting electrochemical properties, optical properties and tunable bandgap.
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The analytical applications of black phosphorus are overviewed.