scispace - formally typeset

Journal ArticleDOI

Uptake routes and toxicokinetics of silver nanoparticles and silver ions in the earthworm Lumbricus rubellus.

01 Oct 2015-Environmental Toxicology and Chemistry (Wiley)-Vol. 34, Iss: 10, pp 2263-2270

TL;DR: The importance of oral exposure questions the application of current metal bioavailability models, which implicitly consider that the dominant route of exposure is via the soil solution, for bioavailability assessment and modeling of metal-based NPs.
Abstract: Current bioavailability models, such as the free ion activity model and biotic ligand model, explicitly consider that metal exposure will be mainly to the dissolved metal in ionic form. With the rise of nanotechnology products and the increasing release of metal-based nanoparticles (NPs) to the environment, such models may increasingly be applied to support risk assessment. It is not immediately clear, however, whether the assumption of metal ion exposure will be relevant for NPs. Using an established approach of oral gluing, a toxicokinetics study was conducted to investigate the routes of silver nanoparticles (AgNPs) and Ag+ ion uptake in the soil-dwelling earthworm Lumbricus rubellus. The results indicated that a significant part of the Ag uptake in the earthworms is through oral/gut uptake for both Ag+ ions and NPs. Thus, sealing the mouth reduced Ag uptake by between 40% and 75%. An X-ray analysis of the internal distribution of Ag in transverse sections confirmed the presence of increased Ag concentrations in exposed earthworm tissues. For the AgNPs but not the Ag+ ions, high concentrations were associated with the gut wall, liver-like chloragogenous tissue, and nephridia, which suggest a pathway for AgNP uptake, detoxification, and excretion via these organs. Overall, the results indicate that Ag in the ionic and NP forms is assimilated and internally distributed in earthworms and that this uptake occurs predominantly via the gut epithelium and less so via the body wall. The importance of oral exposure questions the application of current metal bioavailability models, which implicitly consider that the dominant route of exposure is via the soil solution, for bioavailability assessment and modeling of metal-based NPs.
Topics: Lumbricus rubellus (53%), Bioavailability (51%), Toxicokinetics (50%), Biotic Ligand Model (50%), Silver nanoparticle (50%)

Content maybe subject to copyright    Report

© 2015 SETAC
This version available http://nora.nerc.ac.uk/512228/
NERC has developed NORA to enable users to access research outputs
wholly or partially funded by NERC. Copyright and other rights for material
on this site are retained by the rights owners. Users should read the terms
and conditions of use of this material at
http://nora.nerc.ac.uk/policies.html#access
This document is the author’s final manuscript version of the journal
article, incorporating any revisions agreed during the peer review
process. There may be differences between this and the publisher’s
version. You are advised to consult the publisher’s version if you wish
to cite from this article.
The definitive version is available at http://onlinelibrary.wiley.com/
Article (refereed) - postprint
Diez-Ortiz, Maria; Lahive, Elma; Kille, Peter; Powell, Kate; Morgan, A. John;
Jurkschat, Kerstin; Van Gestel, Cornelis A.M.; Mosselmans, J. Fred W.;
Svendsen, Claus; Spurgeon, David J. 2015. Uptake routes and
toxicokinetics of silver nanoparticles and silver ions in the earthworm
Lumbricus rubellus. Environmental Toxicology and Chemistry, 34 (10).
2263-2270. 10.1002/etc.3036
Contact CEH NORA team at
noraceh@ceh.ac.uk
The NERC and CEH trademarks and logos (‘the Trademarks’) are registered trademarks of NERC in the UK and
other countries, and may not be used without the prior written consent of the Trademark owner.

Ac
c
e
p
te
d
P
r
e
p
r
i
n
t
This article is protected by copyright. All rights reserved
Environmental Toxicology Environmental Toxicology and Chemistry
DOI 10.1002/etc.3036
UPTAKE ROUTES AND TOXICOKINETICS OF SILVER NANOPARTICLES AND
SILVER IONS IN THE EARTHWORM LUMBRICUS RUBELLUS
Running title: Silver uptake in earthworms occurs mainly via oral exposure
MARIA DIEZ-ORTIZ,*† ELMA LAHIVE,† PETER KILLE,‡ KATE POWELL,‡ A. JOHN MORGAN,‡ KERSTIN
JURKSCHAT CORNELIS A.M. VAN GESTEL,|| J. FRED W. MOSSELMANS,# CLAUS SVENDSEN,† and
DAVID J. SPURGEON
Centre for Ecology and Hydrology, Crowmarsh Gifford, Wallingford, Oxfordshire, United
Kingdom
Cardiff School of Biosciences, University of Cardiff, Cardiff, Wales, United Kingdom
§Department of Materials, Oxford University, Yarnton, Oxfordshire, United Kingdom
||
Department of Ecological Science, Faculty of Earth and Life Sciences, VU University,
Amsterdam, The Netherlands
#Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire, United
Kingdom
* Address correspondence to mdiez@leitat.org
This article is protected by copyright. All rights reserved
Submitted 16 June 2014; Returned for Revision 14 August 2014; Accepted 24 April 2015

Ac
c
e
p
te
d
P
r
e
p
r
i
n
t
This article is protected by copyright. All rights reserved
Abstract: Current bioavailability models, such as the free ion activity model and biotic ligand model,
explicitly consider that metal exposure will be mainly to the dissolved metal in ionic form. With the rise
of nanotechnology products and the increasing release of metal-based nanoparticles (NPs) to the
environment, such models may increasingly be applied to support risk assessment. However, it is not
immediately clear whether the assumption of metal ion exposure will be relevant for NPs. Here using
an established approach of oral gluing we have conducted a toxicokinetics study to investigate the
routes of Ag NP and Ag
+
ion uptake in the soil dwelling earthworm Lumbricus rubellus. Results
indicated a significant part of the Ag uptake in the earthworms is through oral/gut uptake for both Ag
+
ions and NPs. Thus, sealing the mouth reduced Ag uptake by between 40-75%. An X-ray analysis of
the internal distribution of Ag in transverse sections confirmed the presence of increased Ag
concentrations in exposed earthworm tissues. For the Ag NPs but not the Ag
+
ions, high concentrations
were associated with the gut wall, liver-like chloragogenous tissue and nephridia, which suggest a
pathway for Ag NP uptake, detoxification and excretion via these organs. Overall our results indicate
that Ag in ionic and NP form is assimilated and internally distributed in earthworms and that this uptake
occurs predominantly via the gut epithelium and less so via the body wall. The importance of oral
exposure questions the application of current metal bioavailability models, which implicitly consider
that the dominant route of exposure is via the soil solution, for bioavailability assessment and modelling
of metal-based NPs. This article is protected by copyright. All rights reserved
Keywords: Silver, Nanoparticles, Exposure route, Uptake, XANES

Ac
c
e
p
te
d
P
r
e
p
r
i
n
t
This article is protected by copyright. All rights reserved
INTRODUCTION
The rapid increase of nanotechnology can be expected to result in increased rates at which
engineered nanoparticles (NPs) are entering the environment. The nature of some common
nanotechnology products, including cosmetics, textiles and personal care products, means that NPs can
be expected to enter wastewater streams, where within sewage systems they may sediment into the
sludge material. The deposition of this waste to land provides a route by which these released NPs may
enter into soil ecosystems [1-3]. Once in the environment, there is the potential for metal-based NPs or
metal ions, that are derived following their solubilisation, to come into contact with organisms and
hence to be accumulated [4-7].
The prevailing ecotoxicology paradigm states that for effects to occur it is necessary for the
material to be taken up into the body, and ultimately reach a target site. For conventional chemicals, the
concepts of toxicokinetics and toxicodynamics are well established as a coherent framework that links
exposure to toxic effects [8]. There is reason to expect that this paradigm will be relevant for NPs,
although some debate remains concerning the extent to which toxicity may be dependent on the
biological interactions resulting once NPs enter tissues and cells. Hence to understand the effects of
NPs it is important to understand key aspects governing uptake into exposed organisms under realistic
conditions.
For organisms that live in soils, NP exposure can be expected to occur through three main
routes. Exposure through air is relevant only for more volatile chemicals and hence for NPs under
normal soil moisture conditions is unlikely to be important. For the remaining two routes, namely
exposure through contact with and transfer across the skin (dermal) and ingestion and transfer across
the gut epithelium (oral), previous studies with conventional chemicals have generally suggested
dermal exposure as the dominant route. This includes pesticides and non-polar organic chemicals in
studies with woodlice and earthworms [9] and for Cd and Zn metal ions in a classic study by Vijver et

Ac
c
e
p
te
d
P
r
e
p
r
i
n
t
This article is protected by copyright. All rights reserved
al. [10] that used surgical glue to inhibit soil ingestion. The latter approach allowed for separate
analysis of dermal uptake with and without the additional inputs derived from ingestion and is adopted
here.
Indications of dermal contact as the dominant route of exposure have underpinned the
development of models, such as the free ion activity model (FIAM) [1] and later the biotic ligand model
(BLM) [12] and related terrestrial bioavailability methods, that link soil solution chemistry and metal
speciation modelling, to passive absorption and surface ligand binding on biological membranes,
notably epidermis and gill and thereafter ultimately to toxic effects [11-14]. The successful application
of these models to explain the effects of variations in media properties on metal toxicity [15-17] has
pointed to the validity of the assumptions inherent in these models, with exposure mainly through
external body surfaces. However, although many studies have used the FIAM and BLM to explain
metal bioavailability and toxicity, not all research has necessarily supported this conjecture regarding
exposure routes. For example, the results of Cain et al. [18] suggested that free ion concentrations
accounted for less than 5% of Cd and Cu accumulation in mayflies in aqueous exposure. While the
identification of a dependence of uptake on gut physiology and microbiome composition [19,20] also
indicates dietary uptake as an important exposure route [21].
Previous studies of soil invertebrate exposure to metal-based NPs have established that these
materials can enter into the tissues either as intact particles or following dissociation to ions [6,22,23].
In cases where the uptake of intact NPs is suggested it is, however, currently unclear how these
materials enter tissues. This uncertainty currently places limits on the development of a modelling
framework that can link NP exposure to uptake and effects, such as for example whether assumption of
dermal uptake are valid. To specifically assess the importance of different potential exposure routes of
NP uptake, we conducted a study to assess the toxicokinetic patterns of Ag uptake in earthworms
exposed to both ionic and NP forms of Ag in soil, using the same oral sealing approaches as used by
Vijver et al [10] to quantify the comparative contributions of both the dermal and oral exposure route.

Citations
More filters

Journal ArticleDOI
01 Feb 2017-EFSA Journal
TL;DR: It is proposed that in‐soil environmental risk assessments are made at in‐ and off‐field scale considering field boundary levels, and a new testing strategy which takes into account the relevant exposure routes for in‐ soil organisms and the potential direct and indirect effects is proposed.
Abstract: Following a request from EFSA, the Panel on Plant Protection Products and their Residues developed an opinion on the science behind the risk assessment of plant protection products for in-soil organisms. The current risk assessment scheme is reviewed, taking into account new regulatory frameworks and scientific developments. Proposals are made for specific protection goals for in-soil organisms being key drivers for relevant ecosystem services in agricultural landscapes such as nutrient cycling, soil structure, pest control and biodiversity. Considering the time-scales and biological processes related to the dispersal of the majority of in-soil organisms compared to terrestrial non-target arthropods living above soil, the Panel proposes that in-soil environmental risk assessments are made at in- and off-field scale considering field boundary levels. A new testing strategy which takes into account the relevant exposure routes for in-soil organisms and the potential direct and indirect effects is proposed. In order to address species recovery and long-term impacts of PPPs, the use of population models is also proposed.

99 citations


Cites background from "Uptake routes and toxicokinetics of..."

  • ...For ‘soft-bodied’ organisms, where water and oxygen is taken up mainly via the skin, soil porewater is considered the most important uptake route for chemicals (Belfroid et al., 1994; EFSA, 2009c; De Silva et al., 2010; Sm ıdov a and Hofman, 2014; Diez-Ortiz et al., 2015)....

    [...]


Journal ArticleDOI
TL;DR: The current state of the art and perspectives for the impact of AgNPs on different organisms present in the environment are reported and recent progress in interpreting uptake, translocation and accumulation mechanisms in different organisms and/or living animals are discussed, as well as the toxicity ofAgNPs and possible tolerance mechanisms in live organisms to cope with their deleterious effects.
Abstract: Silver nanoparticles (AgNPs) have attracted remarkable attention due to their powerful antimicrobial action as well as their particular physicochemical properties. This has led to their application in a wide variety of products with promising results. However, their interaction with the environment and toxicity in live terrestrial or aquatic organisms is still a matter of intense debate. More detailed knowledge is still required about the toxicity of AgNPs, their possible uptake mechanisms and their adverse effects in live organisms. Several studies have reported the interactions and potential negative effects of AgNPs in different organisms. In this review, we report and discuss the current state of the art and perspectives for the impact of AgNPs on different organisms present in the environment. Recent progress in interpreting uptake, translocation and accumulation mechanisms in different organisms and/or living animals are discussed, as well as the toxicity of AgNPs and possible tolerance mechanisms in live organisms to cope with their deleterious effects. Finally, we discuss the challenges of accurate physicochemical characterization of AgNPs and their ecotoxicity in environmentally realistic conditions such as soil and water media.

91 citations


Journal ArticleDOI
Audrey Grech, Céline Brochot, Jean-Lou Dorne1, Nadia Quignot  +2 moreInstitutions (1)
TL;DR: State-of-the-art toxicokinetic tools and models that have been applied to terrestrial and aquatic species relevant to environmental risk assessment of chemicals and a number of chemical classes including plant protection products, metals, persistent organic pollutants, nanoparticles are reviewed.
Abstract: Environmental risk assessment of chemicals for the protection of ecosystems integrity is a key regulatory and scientific research field which is undergoing constant development in modelling approaches and harmonisation with human risk assessment. This review focuses on state-of-the-art toxicokinetic tools and models that have been applied to terrestrial and aquatic species relevant to environmental risk assessment of chemicals. Both empirical and mechanistic toxicokinetic models are discussed using the results of extensive literature searches together with tools and software for their calibration and an overview of applications in environmental risk assessment. These include simple tools such as one-compartment models, multi-compartment models to physiologically-based toxicokinetic (PBTK) models, mostly available for aquatic species such as fish species and a number of chemical classes including plant protection products, metals, persistent organic pollutants, nanoparticles. Data gaps and further research needs are highlighted.

56 citations


Journal ArticleDOI
TL;DR: The nanoparticulate Ag extracted by this method represents the upper limit of the potentially dispersible nanopartsiculate fraction, thus providing a benchmark with which to make quantitative comparisons, while presenting a suspension suitable for a myriad of other characterization analyses.
Abstract: The lack of an efficient and standardized method to disperse soil particles and quantitatively subsample the nanoparticulate fraction for characterization analyses is hindering progress in assessing the fate and toxicity of metallic engineered nanomaterials in the soil environment. This study investigates various soil extraction and extract preparation techniques for their ability to remove nanoparticulate Ag from a field soil amended with biosolids contaminated with engineered silver nanoparticles (AgNPs), while presenting a suitable suspension for quantitative single-particle inductively coupled plasma mass spectroscopy (SP-ICP-MS) analysis. Extraction parameters investigated included reagent type (water, NaNO3, KNO3, tetrasodium pyrophosphate (TSPP), tetramethylammonium hydroxide (TMAH)), soil-to-reagent ratio, homogenization techniques as well as procedures commonly used to separate nanoparticles from larger colloids prior to analysis (filtration, centrifugation, and sedimentation). We assessed the ef...

52 citations


Journal ArticleDOI
TL;DR: Despite the lack of effects in traditional endpoints, Ag NPs and AgNO3 posed changes at lower levels of biological complexity (biochemical, cellular and molecular levels), which represented a relevant alternative for development of non-invasive biomarkers.
Abstract: The current use and development of applications with silver nanoparticles (Ag NPs) could lead to potential inputs of these NPs to soils. Consequently, it is crucial to understand the ecotoxicological risks posed by Ag NPs in the terrestrial compartment. In the present investigation, the effects produced by PVP-PEI coated Ag NPs were assessed in Eisenia fetida earthworms in comparison with the soluble form (AgNO3). Earthworms were exposed for 1, 3 and 14 days to a range of sublethal concentrations of Ag (0, 0.05 and 50mg/kg) and at each exposure time, apart from mortality and weight loss of individuals, metallothionein (MT) protein concentration and catalase (CAT) activity were quantified in earthworm tissues. In addition, cellular and molecular level endpoints (cell viability, absolute and relative trophic indices and transcription levels of catalase-cat- and metallothionein-mt-) were measured in coelomocytes extruded from exposed earthworms. Despite the lack of effects in traditional endpoints (mortality and weight loss), Ag NPs and AgNO3 posed changes at lower levels of biological complexity (biochemical, cellular and molecular levels). Both Ag forms induced similar changes in the metal detoxification mechanism (MT, mt) and in the antioxidant response system (CAT, cat) of E. fetida. In contrast, Ag form dependant cytotoxicity and subpopulation ratio alterations (eleocytes/amoebocytes) were recorded in extruded coelomocytes. Complementarily, the use of coelomocytes to assess molecular level endpoints represented a relevant alternative for development of non-invasive biomarkers.

38 citations


Cites background from "Uptake routes and toxicokinetics of..."

  • ...…involved the effects of Ag NPs on terrestrial organisms, the number of studies carried out with earthworms has increased during the last five years (Diez-Ortiz et al., 2015a, 2015b; García-Velasco et al., 2016; Gomes et al., 2013, 2015; Hayashi et al., 2012; Kwak and An, 2015; Shoults-Wilson et…...

    [...]


References
More filters

Journal ArticleDOI
TL;DR: The results of this study indicate that risks to aquatic organisms may currently emanate from nano- Ag, nano-TiO(2), and nano-ZnO in sewage treatment effluents for all considered regions and for nano-Ag in surface waters.
Abstract: Engineered nanomaterials (ENM) are already used in many products and consequently released into environmental compartments. In this study, we calculated predicted environmental concentrations (PEC) based on a probabilistic material flow analysis from a life-cycle perspective of ENM-containing products. We modeled nano-TiO2, nano-ZnO, nano-Ag, carbon nanotubes (CNT), and fullerenes for the U.S., Europe and Switzerland. The environmental concentrations were calculated as probabilistic density functions and were compared to data from ecotoxicological studies. The simulated modes (most frequent values) range from 0.003 ng L−1 (fullerenes) to 21 ng L−1 (nano-TiO2) for surface waters and from 4 ng L−1 (fullerenes) to 4 μg L−1 (nano-TiO2) for sewage treatment effluents. For Europe and the U.S., the annual increase of ENMs on sludge-treated soil ranges from 1 ng kg−1 for fullerenes to 89 μg kg−1 for nano-TiO2. The results of this study indicate that risks to aquatic organisms may currently emanate from nano-Ag, n...

2,061 citations


"Uptake routes and toxicokinetics of..." refers background in this paper

  • ...As the concentrations used in the present study were above current anticipated environmental concentrations [32], they need to be validated at lower concentrations....

    [...]


Journal ArticleDOI
Vicente A. Solé1, Emmanuel Papillon1, Marine Cotte1, Ph. Walter2  +1 moreInstitutions (2)
Abstract: A user friendly program for X-ray fluorescence analysis has been developed at the European Synchrotron Radiation Facility The program allows interactive as well as batch processing of large data sets and it is particularly well suited for X-ray imaging Its implementation of a complete description of the M shell is particularly helpful for analysis of data collected at low energies The code is platform independent (Linux, Windows, MacOS X, Solaris …) and it is freely available for non-commercial use Description of the algorithms used and practical examples are presented

1,340 citations


"Uptake routes and toxicokinetics of..." refers background in this paper

  • ...1 [29], to remove the signal observed from the argon Kb line and produce the Ag X-ray fluorescence map....

    [...]


01 Jan 2001-
TL;DR: The biotic ligand model of acute metal toxicity to aquatic organisms is based on the idea that mortality occurs when the metal-biotic ligand complex reaches a critical concentration, a generalization of the free ion activity model that relates toxicity to the concentration of the divalent metal cation.
Abstract: The biotic ligand model (BLM) of acute metal toxicity to aquatic organisms is based on the idea that mortality occurs when the metal-biotic ligand complex reaches a critical concentration. For fish, the biotic ligand is either known or suspected to be the sodium or calcium channel proteins in the gill surface that regulate the ionic composition of the blood. For other organisms, it is hypothesized that a biotic ligand exists and that mortality can be modeled in a similar way. The biotic ligand interacts with the metal cations in solution. The amount of metal that binds is determined by a competition for metal ions between the biotic ligand and the other aqueous ligands, particularly dissolved organic matter (DOM), and the competition for the biotic ligand between the toxic metal ion and the other metal cations in solution, for example, calcium. The model is a generalization of the free ion activity model that relates toxicity to the concentration of the divalent metal cation. The difference is the presence of competitive binding at the biotic ligand, which models the protective effects of other metal cations, and the direct influence of pH. The model is implemented using the Windermere humic aqueous model (WHAM) model of metal-DOM complexation. It is applied to copper and silver using gill complexation constants reported by R. Playle and coworkers. Initial application is made to the fathead minnow data set reported by R. Erickson and a water effects ratio data set by J. Diamond. The use of the BLM for determining total maximum daily loadings (TMDLs) and for regional risk assessments is discussed within a probabilistic framework. At first glance, it appears that a large amount of data are required for a successful application. However, the use of lognormal probability distributions reduces the required data to a manageable amount. Keywords—Bioavailability Metal toxicity Metal complexation Risk assessment

1,142 citations


Journal ArticleDOI
Abstract: The biotic ligand model (BLM) of acute metal toxicity to aquatic organisms is based on the idea that mortality occurs when the metal-biotic ligand complex reaches a critical concentration. For fish, the biotic ligand is either known or suspected to be the sodium or calcium channel proteins in the gill surface that regulate the ionic composition of the blood. For other organisms, it is hypothesized that a biotic ligand exists and that mortality can be modeled in a similar way. The biotic ligand interacts with the metal cations in solution. The amount of metal that binds is determined by a competition for metal ions between the biotic ligand and the other aqueous ligands, particularly dissolved organic matter (DOM), and the competition for the biotic ligand between the toxic metal ion and the other metal cations in solution, for example, calcium. The model is a generalization of the free ion activity model that relates toxicity to the concentration of the divalent metal cation. The difference is the presence of competitive binding at the biotic ligand, which models the protective effects of other metal cations, and the direct influence of pH. The model is implemented using the Windermere humic aqueous model (WHAM) model of metal-DOM complexation. It is applied to copper and silver using gill complexation constants reported by R. Playle and coworkers. Initial application is made to the fathead minnow data set reported by R. Erickson and a water effects ratio data set by J. Diamond. The use of the BLM for determining total maximum daily loadings (TMDLs) and for regional risk assessments is discussed within a probabilistic framework. At first glance, it appears that a large amount of data are required for a successful application. However, the use of lognormal probability distributions reduces the required data to a manageable amount.

1,089 citations


Journal ArticleDOI
TL;DR: The ecotoxicological literature shows that concentrations of Ag NPs below the current and future PECs, as low as just a few ng L(-1), can affect prokaryotes, invertebrates and fish indicating a significant potential, though poorly characterised, risk to the environment.
Abstract: This review summarises and evaluates the present knowledge on the behaviour, the biological effects and the routes of uptake of silver nanoparticles (Ag NPs) to organisms, with considerations on the nanoparticle physicochemistry in the ecotoxicity testing systems used. Different types of Ag NP syntheses, characterisation techniques and predicted current and future concentrations in the environment are also outlined. Rapid progress in this area has been made over the last few years, but there is still a critical lack of understanding of the need for characterisation and synthesis in environmental and ecotoxicological studies. Concentration and form of nanomaterials in the environment are difficult to quantify and methodological progress is needed, although sophisticated exposure models show that predicted environmental concentrations (PECs) for Ag NPs in different environmental compartments are at the range of ng L(-1) to mg kg(-1). The ecotoxicological literature shows that concentrations of Ag NPs below the current and future PECs, as low as just a few ng L(-1), can affect prokaryotes, invertebrates and fish indicating a significant potential, though poorly characterised, risk to the environment. Mechanisms of toxicity are still poorly understood although it seems clear that in some cases nanoscale specific properties may cause biouptake and toxicity over and above that caused by the dissolved Ag ion. This review concludes with a set of recommendations for the advancement of understanding of the role of nanoscale silver in environmental and ecotoxicological research.

1,019 citations


"Uptake routes and toxicokinetics of..." refers background in this paper

  • ...Models of ionicmetal toxicity, such as the BLM, have been used to account for soil and water chemistry influences on toxicity, and it has been proposed that such models could be useful for predicting the toxicity of metals and metal oxide NPs [37]....

    [...]


Performance
Metrics
No. of citations received by the Paper in previous years
YearCitations
20221
202110
20206
20195
20187
20179