Abstract
In this mini-review, we briefly describe certain recently developed applications of the surface-enhanced Raman spectroscopy (SERS) for determining various biochemically (especially medically) important species from ones as simple as hydrogen cations to those as complex as specific DNA fragments. We present a SERS analysis of species whose characterization is important to our understanding of various mechanisms in the human body and to show its potential as an alternative for methods routinely used in diagnostics and clinics. Furthermore, we explain how such SERS-based sensors operate and point out future prospects in this field.
Introduction
Surface-enhanced Raman scattering (SERS) is one of the most sensitive analytical tools known—in some cases, it is possible to record a high-quality SERS spectrum dominated by the contribution of even a single molecule (Kneipp et al., ). The SERS spectroscopy is therefore considered a very promising option for routine analytical techniques used in medical, biochemical, environmental, and food analyses.
In this mini-review, we briefly describe certain recently developed applications of SERS spectroscopy for characterizing biochemically (and especially medically) important compounds. We also outline the basic theoretical background of the SERS effect and discuss potential future applications of SERS in this field. We hope that this paper will be useful for researchers who are planning to enter this fascinating field.
Surface-Enhanced Raman Spectroscopy
In the 1970's, it was observed that the Raman signal generated by molecules adsorbed on some nanostructured materials was increased by many orders of magnitude. This phenomenon (called SERS) was explained as a result of the synergistic cooperation of two mechanisms based on: (i) the excitation of the localized surface plasmons and (ii) the chemical interactions (see Supplementary Material). When nanostructures formed from materials with a negative real and a small positive imaginary dielectric constant at a given excitation frequency (e.g., Au, Ag, and Cu) interact with an electromagnetic wave, the collective oscillations of surface conduction electrons (called surface plasmons) are induced, that generate an additional electric field in close proximity to the illuminated nanostructure. In the case of homogeneous plasmonic nanostructures, the strongest enhancement of the electromagnetic field occurs at the sharp apexes and edges; in a case of agglomerates or aggregates of plasmonic nanostructures, very large field enhancement is observed in the slits between nanograins—such places are called “hot spots.” In the SERS spectroscopy, the increase in the efficiency of the Raman signal generated is roughly proportional to the fourth power of the field enhancement (Aroca, ; Kudelski, ), and because of this fourth power dependence, very large SERS enhancement factors can be achieved, making the SERS spectroscopy one of the most sensitive analytical tools. The chemical mechanism of SERS involves the hybridization of orbitals of the adsorbed molecules with the orbitals of metal, which facilitates resonance Raman scattering. The chemical mechanism is only important for molecules interacting directly with the metal surface, and therefore, is usually not operating in SERS sensors.
Surface-Enhanced Raman Scattering for (Bio)Medical Applications
The SERS spectroscopy has been widely used for analyzing various biosamples, including DNA (Pyrak et al., ; Zhang et al., ), RNA (Lee et al., ; Han et al., ), cancer markers (Choi et al., ), bacteria (Andrei et al., ), viruses (Chen et al., ), genes (Vo-Dinh et al., ), drugs (Jaworska et al., ), pathological markers on cellular membranes and tissues (Wallace and Masson, ), other biomolecules, ion concentrations, and redox potential in cells (Jaworska et al., ), and even in vivo SERS measurements on mice (Wen et al., ). In many cases, the results obtained are at a level of detection not achievable by other analytical methods. However, the Raman spectroscopy still remains to be included among the routinely applied biochemical methods, such as UV–VIS spectroscopy, fluorescence, and PCR. Recent experiments demonstrate the huge potential of this technique, and it is possible that at some point the SERS spectroscopy will be successfully (routinely) performed on medical samples. Later, we discuss the “hottest” examples of the SERS applications, divided into two groups: label-free detection with easy sample preparation and complicated data analysis, and SERS-based nanosensors, which very often produce zero-one results.
Label-Free Direct Detection via SERS
The experimental procedure in the label-free SERS detection requires only the adsorption of the analyzed sample directly on the nanostructured plasmonic surface. The obtained spectra are usually, however, difficult to analyze due to the low signal-to-noise ratio and small differences between obtained spectra (e.g., spectra of healthy and cancer cells or different bacteria types). The example of the label-free experiment is shown in Figure 1A. Although this approach is less specific than a nanosensor-based approach, it still provides quite promising results when combined with highly advanced data analysis. Some of the latest examples of label-free SERS applications are: measuring the urine of a patient to detect and monitor the risk of a relapse of prostate cancer (Ma et al., ); characterizing the hepatitis C viral (HCV) RNA extracted from different blood samples of patients with HCV infection (Nasir et al., ); diagnosing ulcerative colitis in blood plasma (Tefas et al., ); and assessing the effectiveness of antiplatelet therapy (Zyubin et al., ). In label-free experiments, unfortunately, it is usually impossible to find a new band differentiating two sets of samples. However, by means of chemometrics, we can find subtle differences within the relative intensity or shape of bands. On the one hand, such a classification is not definite, and it is hard to say if this could ever be possible. On the other hand, SERS is one of the most sensitive techniques for detecting compounds, such as neurotransmitters at the attomolar level, and has significant advantages over routine methods (Lee et al., ). The label-free detection of the dengue virus in blood samples is also an impressive example of a fast SERS-based procedure that requires only a very small sample (5 μl) (Gahlaut et al., ). The SERS spectra were recorded from samples placed on nanostructured silver substrates; the samples from patients with dengue positive contained dengue-specific immunoglobulin (IgM) antibodies. Also, many bacteria have been detected using a label-free approach. For example, a SERS-active platform based on a polymer nanofiber mat has been shown to be a reliable SERS surface for detecting Staphylococcus aureus, Pseudomonas aeruginosa, and Salmonella typhimurium in blood plasma at a concentration of 103 colony forming unit/ml (Witkowska et al., ). Also, the identification of the cellular composition of Gram-positive and Gram-negative bacteria by using mesoporous silicon-based substrates decorated with silver nanoparticles was carried out, and spectral differences were noticeable regarding their different cell cycles (Paccotti et al., ).
Figure 1
In the above-described experiments, SERS proved itself as a reliable methodology. However, the label-free approach is limited by the fact that not all the changes in characteristics of a sample are connected with significant spectral differences. Therefore, in our opinion, the most promising way of introducing SERS into clinics is to prepare sensors that make it possible to detect specific molecules at an ultra-low limit of detection, instead of using the easier but less reliable label-free approach.
Surface-Enhanced Raman Scattering-Based Nanosensors
An alternative way of using SERS is to prepare nanosensors that contain Raman reporter molecules (RRMs). These are mainly organic dyes such as rhodamine 6G or malachite green, which have an extremely high cross-section for Raman scattering. We can combine these molecules directly with the substance that we want to detect and record the Raman signal of the RRM instead of, for example, the Raman signal of the proteins, which is much lower. An example of such a sensor allowing the distinguishing of over-expressing specific markers of normal and cancer cells can be seen in Figure 1B. Briefly, metal nanoparticles labeled with RRM and specific antibodies interact specifically with the markers expressed on the cellular membrane. The SERS spectra of the cells are recorded (in the form of a map), and the signal from the RRM indicates the presence of the selected marker on the cell (Lee et al.,
Therapeutic Drug Monitoring
Therapeutic drug monitoring (TDM) involves assessing the drug concentration in a biological matrix (most commonly plasma or serum) at a known time related to administration and interpreting these concentrations in terms of relevant clinical parameters (target range and pharmacokinetics of the drug) (Jaworska et al.,
Detection of Glucose
Another topic of interest in medical diagnostics is monitoring the level of glucose. Nowadays, this requires several measurements of blood daily, which causes discomfort, pain, and the risk of contamination. Developing a less invasive, continuous glucose monitoring device would have a great impact on 415 million diabetics worldwide. For example, a low-cost SERS sensor for the in situ intradermal detection of glucose was developed by Ju et al. (
Disease Markers
The traditional tissue biopsy is limited in enhancing our understanding of the heterogeneity and dynamic evolution of tumors (Zhang et al.,
Intracellular Environment
Apart from monitoring specific markers on the cellular membrane, SERS can be used to monitor the intracellular environment. It has been applied to measure, for example, the concentration of different ions (Mg2+ and Ca2+), pH, and redox potential. Usually, these values are measured indirectly by measuring the SERS spectra of specific reporter molecules, which change as a function of the concentration of the species of interest. For example, this approach was applied to monitor microRNA-21 and telomerase in cells (Liu et al.,
The SERS nanosensors can be successfully applied for measuring intracellular pH, even in selected organelles (lysosomes, nuclei, and mitochondria). Plasmonic metal nanoparticles labeled with pH-sensitive molecules and specific organelle-targeting peptides can be easily delivered into organelles, and the SERS mapping of the cell provides information about the pH in a specified location (Shen et al.,
Surface-Enhanced Raman Scattering Detection of Bacteria
Another interesting example of the potential of SERS is the detection of bacteria. Bi et al. (
Surface-Enhanced Raman Scattering Combined With Other Methods
Interesting examples showing the potential of SERS in clinical applications include those in which SERS is combined with lateral flow assays. With this approach, the experimental procedure involves the preparation of strips, similar to pregnancy tests, which contain specific aptamers, antibodies, antigens, and SERS substrates that interact with the sample. Then, the appearance of the characteristic SERS signal confirms the presence of specific markers (Wang et al.,
Figure 2

(A) Schematic illustration of a paper-based lateral flow strip. Reprinted with permission from Gao et al. (
Also, the use of paper substrates significantly lowers the cost of the SERS substrates, especially when combined with microfluidics, where only a small volume of sample is needed. This approach was successfully applied by Torul et al. (
The SERS spectroscopy is very often combined with PCR. PCR is used to amplify the target gene to a detectable level, and it is currently difficult to imagine detecting mutations in DNA and RNA without using it. For example, Lee et al. (
Conclusions and Future Perspectives
In this mini-review, we briefly outlined recent advances in the application of SERS for the detection and determination of certain biochemically (and especially medically) important species. We expect further intensive development of biochemical and chemical SERS sensors, especially DNA/RNA ones that can be used for the early identification of cancer-connected DNA mutations and various bacteria and viruses. Moreover, DNA sensors developed for medical applications may be further applied in other areas, for example, to verify the authenticity of some food products. While many working SERS biosensors have already been developed, many aspects still need to be significantly improved before commercialization can occur, for example, improving the reproducibility of SERS substrates. We hope that the examples presented in this mini-review will convince readers that the SERS spectroscopy is a very promising option for some of the routine analytical techniques used in medical, biochemical, and biological analyses.
Statements
Author contributions
AK and AS carried out literature searching and wrote the manuscript. All authors contributed to the manuscript and approved it for publication.
Acknowledgments
AK and AS thank the Faculty of Chemistry, University of Warsaw for the financial support.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2021.664134/full#supplementary-material
References
1
AndreiC.-C.MoraillonA.LarquetE.PotaraM.AstileanS.JakabE.et al. (2021). SERS characterization of aggregated and isolated bacteria deposited on silver-based substrates. Anal. Bioanal. Chem.413, 1417–1428. 10.1007/s00216-020-03106-5
2
ArocaR. (2006). Surface-Enhanced Vibrational Spectroscopy. Chichester: John Wiley & Sons. 10.1002/9780470035641
3
BiL.WangX.CaoX.LiuL.BaiC.ZhengQ.et al. (2020). SERS-active Au@Ag core-shell nanorod (Au@AgNR) tags for ultrasensitive bacteria detection and antibiotic-susceptibility testing. Talanta220:121397. 10.1016/j.talanta.2020.121397
4
ChenH.DasA.BiL.ChoiN.MoonJ.-I.WuY.et al. (2020). Recent advances in surface-enhanced Raman scattering-based microdevices for point-of-care diagnosis of viruses and bacteria. Nanoscale12, 21560–21570. 10.1039/D0NR06340A
5
ChoiN.DangH.DasA.SimM. S.ChungI. Y.ChooJ. (2020). SERS biosensors for ultrasensitive detection of multiple biomarkers expressed in cancer cells. Biosens. Bioelectr.164:112326. 10.1016/j.bios.2020.112326
6
GahlautS. K.SavargaonkarD.SharanC.YadavS.MishraP.SinghJ. P. (2020). SERS platform for dengue diagnosis from clinical samples employing a hand held Raman spectrometer. Anal. Chem.92, 2527–2534. 10.1021/acs.analchem.9b04129
7
GaoX.BoryczkaJ.KasaniS.WuN. (2021a). Enabling direct protein detection in a drop of whole blood with an “on-strip” plasma separation unit in a paper-based lateral flow strip. Anal. Chem.93, 1326–1332. 10.1021/acs.analchem.0c02555
8
GaoX.BoryczkaJ.ZhengP.KasaniS.YangF.Engler-ChiurazziE. B.et al. (2021b). A “hot spot” -enhanced paper lateral flow assay for ultrasensitive detection of traumatic brain injury biomarker S-100β in blood plasma. Biosens. Bioelectr.177:112967. 10.1016/j.bios.2021.112967
9
HanY.QiangL.GaoY.GaoJ.HeQ.LiuH.et al. (2021). Large-area surface-enhanced Raman spectroscopy substrate by hybrid porous GaN with Au/Ag for breast cancer miRNA detection. Appl. Surface Sci.541:148456. 10.1016/j.apsusc.2020.148456
10
JaworskaA.FornasaroS.SergoV.BonifacioA. (2016). Potential of surface enhanced Raman spectroscopy (SERS) in therapeutic drug monitoring (TDM). A critical review. Biosensors6:47. 10.3390/bios6030047
11
JaworskaA.MalekK.KudelskiA. (2021). Intracellular pH – Advantages and pitfalls of surface-enhanced Raman scattering and fluorescence microscopy – a review. Spectrochim. Acta A Mol. Biomol. Spectros.251:119410. 10.1016/j.saa.2020.119410
12
JuJ.HsiehC.-M.TianY.KangJ.ChiaR.ChangH.et al. (2020). Surface enhanced Raman spectroscopy based biosensor with a microneedle array for minimally invasive in vivo glucose measurements. ACS Sensors5, 1777–1785. 10.1021/acssensors.0c00444
13
KimK.Kashefi-KheyrabadiL.JoungY.KimK.DangH.ChavanS. G.et al. (2021). Recent advances in sensitive surface-enhanced Raman scattering-based lateral flow assay platforms for point-of-care diagnostics of infectious diseases. Sensors Actuat. B Chem.329:129214. 10.1016/j.snb.2020.129214
14
KneippJ.KneippH.KneippK. (2008). SERS—a single-molecule and nanoscale tool for bioanalytics. Chem. Soc. Rev.37, 1052–1060. 10.1039/b708459p
15
KudelskiA. (2009). Raman spectroscopy of surfaces. Surface Sci.603, 1328–1334. 10.1016/j.susc.2008.11.039
16
LeeH. G.ChoiW.YangS. Y.KimD.-H.ParkS.-G.LeeM.-Y.et al. (2021a). PCR-coupled paper-based surface-enhanced Raman scattering (SERS) sensor for rapid and sensitive detection of respiratory bacterial DNA. Sensors Actuat. B Chem.326:128802. 10.1016/j.snb.2020.128802
17
LeeJ. U.KimW. H.LeeH. S.ParkK. H.SimS. J. (2019). Quantitative and specific detection of exosomal miRNAs for accurate diagnosis of breast cancer using a surface-enhanced Raman scattering sensor based on plasmonic head-flocked gold nanopillars. Small15:1804968. 10.1002/smll.201970091
18
LeeS.KimS.ChooJ.ShinS. Y.LeeY. H.ChoiH. Y.et al. (2007). Biological imaging of HEK293 cells expressing PLCγ1 using surface-enhanced Raman microscopy. Anal. Chem.79, 916–922. 10.1021/ac061246a
19
LeeW.KangB.-H.YangH.ParkM.KwakJ. H.ChungT.et al. (2021b). Spread spectrum SERS allows label-free detection of attomolar neurotransmitters. Nat. Commun.12:159. 10.1038/s41467-020-20413-8
20
LiJ.LiuH.RongP.ZhouW.GaoX.LiuD. (2018). A universal strategy for the one-pot synthesis of SERS tags. Nanoscale10, 8292–8297. 10.1039/C8NR00564H
21
LinD.QinT.WangY.SunX.ChenL. (2014). Graphene oxide wrapped SERS tags: multifunctional platforms toward optical labeling, photothermal ablation of bacteria, and the monitoring of killing effect. ACS Appl. Mater. Interfaces6, 1320–1329. 10.1021/am405396k
22
LiuC.XuT.ChengG.ZhangX. (2021). Target-triggered regioselective assembly of nanoprobes for Raman imaging of dual cancer biomarkers in living cells. Sensors Actuat. B Chem.330:129319. 10.1016/j.snb.2020.129319
23
MaY.ChiJ.ZhengZ.AttygalleA.KimI. Y.DuH. (2021). Therapeutic prognosis of prostate cancer using surface-enhanced Raman scattering of patient urine and multivariate statistical analysis. J. Biophoton.14:e202000275. 10.1002/jbio.202000275
24
MarkinaN. E.UstinovS. N.ZakharevichA. M.MarkinA. V. (2020). Copper nanoparticles for SERS-based determination of some cephalosporin antibiotics in spiked human urine. Anal. Chimica Acta1138, 9–17. 10.1016/j.aca.2020.09.016
25
NasirS.MajeedM. I.NawazH.RashidN.AliS.FarooqS.et al. (2020). Surface enhanced Raman spectroscopy of RNA samples extracted from blood of Hepatitis C patients for quantification of viral loads. Photodiagnosis Photodyn. Ther. 2020:102152. 10.1016/j.pdpdt.2020.102152
26
NimaZ. A.MahmoodM.XuY.MustafaT.WatanabeF.NedosekinD. A.et al. (2014). Circulating tumor cell identification by functionalized silver-gold nanorods with multicolor, super-enhanced SERS and photothermal resonances. Sci. Rep.4:4752. 10.1038/srep04752
27
PaccottiN.BoschettoF.HoriguchiS.MarinE.Chiad,òA.NovaraC.et al. (2018). Label-free SERS discrimination and in situ analysis of life cycle in Escherichia coli and Staphylococcus epidermidis. Biosensors8:131. 10.3390/bios8040131
28
PyrakE.KrajczewskiJ.KowalikA.KudelskiA.JaworskaA. (2019). Surface enhanced Raman spectroscopy for DNA biosensors—how far are we?Molecules24:4423. 10.3390/molecules24244423
29
RestainoS. M.WhiteI. M. (2018). Real-time multiplexed PCR using surface enhanced Raman spectroscopy in a thermoplastic chip. Lab. Chip18, 832–839. 10.1039/C7LC01227F
30
Sánchez-PurràM.Roig-SolvasB.Rodriguez-QuijadaC.LeonardoB. M.Hamad-SchifferliK. (2018). Reporter selection for nanotags in multiplexed surface enhanced Raman spectroscopy assays. ACS Omega3, 10733–10742. 10.1021/acsomega.8b01499
31
ShenY.LiangL.ZhangS.HuangD.ZhangJ.XuS.et al. (2018). Organelle-targeting surface-enhanced Raman scattering (SERS) nanosensors for subcellular pH sensing. Nanoscale10, 1622–1630. 10.1039/C7NR08636A
32
TefasC.MărgineanR.TomaV.PetrushevB.FischerP.TantăuM.et al. (2021). Surface-enhanced Raman scattering for the diagnosis of ulcerative colitis: will it change the rules of the game?Anal. Bioanal. Chem.413, 827–838. 10.1007/s00216-020-03036-2
33
TorulH.ÇiftçiH.ÇetinD.SuludereZ.BoyaciI. H.TamerU. (2015). Paper membrane-based SERS platform for the determination of glucose in blood samples. Anal. Bioanal. Chem.407, 8243–8251. 10.1007/s00216-015-8966-x
34
Vo-DinhT.AllainL. R.StokesD. L. (2002). Cancer gene detection using surface-enhanced Raman scattering (SERS). J. Raman Spectrosc.33, 511–516. 10.1002/jrs.883
35
WallaceG. Q.MassonJ.-F. (2020). From single cells to complex tissues in applications of surface-enhanced Raman scattering. Analyst145, 7162–7185. 10.1039/D0AN01274B
36
WangR.KimK.ChoiN.WangX.LeeJ.JeonJ. H.et al. (2018). Highly sensitive detection of high-risk bacterial pathogens using SERS-based lateral flow assay strips. Sensors Actuat. B Chem.270, 72–79. 10.1016/j.snb.2018.04.162
37
WangW.ZhaoF.LiM.ZhangC.ShaoY.TianY. (2019). A SERS optophysiological probe for the real-time mapping and simultaneous determination of the carbonate concentration and pH value in a live mouse brain. Angewandte Chem. Int. Ed.58, 5256–5260. 10.1002/anie.201814286
38
WangZ.ZongS.WuL.ZhuD.CuiY. (2017). SERS-activated platforms for immunoassay: probes, encoding methods, and applications. Chem. Rev.117, 7910–7963. 10.1021/acs.chemrev.7b00027
39
WeeE. J. H.WangY.TsaoS. C.-H.TrauM. (2016). Simple, sensitive and accurate multiplex detection of clinically important melanoma DNA mutations in circulating tumour DNA with SERS nanotags. Theranostics6, 1506–1513. 10.7150/thno.15871
40
WenC.ChenH.GuoX.LinZ.ZhangS.ShenX.-C.et al. (2021). Lysosome-targeted gold nanotheranostics for in situ SERS monitoring pH and multimodal imaging-guided phototherapy. Langmuir37, 569–577. 10.1021/acs.langmuir.0c03290
41
WitkowskaE.SzymborskiT.KamińskaA.WalukJ. (2017). Polymer mat prepared via ForcespinningTM as a SERS platform for immobilization and detection of bacteria from blood plasma. Mater. Sci. Eng. C71, 345–350. 10.1016/j.msec.2016.10.027
42
ZhangD.HuangL.LiuB.NiH.SunL.SuE.et al. (2018). Quantitative and ultrasensitive detection of multiplex cardiac biomarkers in lateral flow assay with core-shell SERS nanotags. Biosens. Bioelectr.106, 204–211. 10.1016/j.bios.2018.01.062
43
ZhangJ.DongY.ZhuW.XieD.ZhaoY.YangD.et al. (2019a). Ultrasensitive detection of circulating tumor DNA of lung cancer via an enzymatically amplified SERS-based frequency shift assay. ACS Appl. Mater. Interfaces11, 18145–18152. 10.1021/acsami.9b02953
44
ZhangX.LiuC.PeiY.SongW.ZhangS. (2019b). Preparation of a novel Raman probe and its application in the detection of circulating tumor cells and exosomes. ACS Appl. Mater. Interfaces11, 28671–28680. 10.1021/acsami.9b09465
45
ZhangY.MiX.TanX.XiangR. (2019c). Recent progress on liquid biopsy analysis using surface-enhanced Raman spectroscopy. Theranostics9, 491–525. 10.7150/thno.29875
46
ZyubinA.RafalskiyV.TcibulnikovaA.MoiseevaE.MatveevaK.TsapkovaA.et al. (2020). Surface-enhanced Raman spectroscopy for antiplatelet therapy effectiveness assessment. Laser Phys. Lett.17:045601. 10.1088/1612-202X/ab7be5
Summary
Keywords
surface enhanced Raman spectroscopy, DNA, proteins, cells, tissues, cancer
Citation
Szaniawska A and Kudelski A (2021) Applications of Surface-Enhanced Raman Scattering in Biochemical and Medical Analysis. Front. Chem. 9:664134. doi: 10.3389/fchem.2021.664134
Received
04 February 2021
Accepted
09 April 2021
Published
07 May 2021
Volume
9 - 2021
Edited by
Christa Brosseau, Saint Mary's University, Canada
Reviewed by
Daniela Iacopino, University College Cork, Ireland; Lingxin Chen, Yantai Institute of Coastal Zone Research (CAS), China
Updates

Check for updates
Copyright
© 2021 Szaniawska and Kudelski.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Aleksandra Szaniawska aleksandra.szaniawska@uw.edu.plAndrzej Kudelski akudel@chem.uw.edu.pl
This article was submitted to Analytical Chemistry, a section of the journal Frontiers in Chemistry
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.