Gallik asit tayini için elektrokimyasal sensör olarak yarıiletken nanokompozit yapılarının geliştirilmesi
Development of semiconductor nanocomposite structures as electrochemical sensors for the determination of gallic acid
Yazar: İREM SARIKAYA
Danışman: PROF. DR. MAHMUT ÖZACAR
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Ana Bilim Dalı / Fizikokimya Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Kimya = Chemistry

Özet:
Antioksidanlar, insan vücudundaki serbest radikalleri temizleyerek birçok doğal
hastalığı iyileştirir. Bu nedenle, antioksidan içeren gıdaların tüketimi artmıştır.
Ancak, yapay antioksidanların aşırı alımı kanser gibi hastalıklara neden olabilir. Bu
yüzden, doğal kaynaklardan elde edilen antioksidanlar tercih edilmektedir.
Gallik asit (GA), doğal örneklerde bulunan bir antioksidandır. Bazen 3,4,5-
trihidroksibenzoik asit olarak da bilinen gallik asit, bir polifenol grubu üyesidir.
C₆H₂(OH)₃CO₂H moleküllerinden oluşur. Gallik asit, sarı-beyaz renkte olup
kristal bir yapıya sahiptir. Suda kolayca çözünür ve eter, alkol, gliserol gibi
maddelerde de çözünebilir, ancak benzen ve kloroformda çözünmez. Nar, muz,
böğürtlen, yaban mersini, avokado, frenk üzümü, mango, dut ve sumak gibi birçok
meyvede; ayrıca yeşil çay, sumak, fındık, meşe ve kestane ağaçları gibi bitkisel
ürünlerde ve bazı ağaç kabuklarında gallik asit bulunur.
Bu çalışmada, gallik asit (GA) tayini için çinko oksit (ZnO), çinko sülfür (ZnS) ve
çinko selenid (ZnSe) yarı iletkenleri kullanılarak elektrokimyasal sensörler
geliştirildi ve elektrot üretimi için camsı karbon elektrot (GCE) kullanıldı. GA tayini,
0.1 M fosfat tamponu (PBS) (pH 7.4) içeren çözeltilerde farklı GA
konsantrasyonlarında hazırlanan GCE/ZnS, GCE/ZnSe, GCE/ZnO çalışma
elektrotları kullanılarak yapıldı. Oluşturulan GA sensörleri, 10 µM ile 1000 µM
arasında geniş bir doğrusal yanıt aralığı gösterdi. GCE/ZnS, GCE/ZnSe, GCE/ZnO
elektrotlarıyla geliştirilen elektrokimyasal sensörler, GA tayini için yüksek
hassasiyet, iyi tekrarlanabilirlik, yüksek seçicilik ve uzun vadeli stabilite gösterdi.
Geliştirilen GA sensörü, GCE/ZnS, GCE/ZnSe, GCE/ZnO elektrotlarında farklı LOD
ve LOQ değerleri gösterdi ve -0.80 V, 1.00 V'da 10-100 µM GA konsantrasyon
aralığında doğrusal bir yanıt verdi. LOD-LOQ değerleri sırasıyla 9.03 µM-27.36 µM,
6.33 µM-19.203 µM, 5.55 µM-16.84 µM olarak gözlemlendi.
Elektrokimyasal sensöre ek olarak, Folin-Ciocalteu yöntemiyle spektrofotometrik
olarak da GA tayini yapıldı ve sonuçlar karşılaştırıldı. Folin-Ciocalteu yöntemiyle
elde edilen sensörün LOD değeri 5.48 µM ve LOQ değeri 16.62 µM olarak bulundu.
Ayrıca, gerçek kiraz suyu ve ceviz örneklerinde GA tayini yapıldı ve geri kazanım
değerleri %98.15-101.84 aralığında belirlendi. GA, Folin-Ciocalteu yöntemi ve
elektrokimyasal sensör kullanılarak spektrofotometrik olarak belirlendi ve sonuçlar
karşılaştırıldı. GA ayrıca vişne suyu ve ceviz gibi gerçek örneklerde de belirlendi.

Abstract:
Free radicals are molecules or atoms with one or more unpaired electrons outside their
orbitals. These unpaired electrons are denoted by a dot adjacent to the atom or
molecule. Free radicals, in their quest to complete their missing electrons, exhibit high
reactivity and a strong tendency to form bonds and compounds.
Bond formation necessitates the presence of two electrons with spins oriented in
opposite directions. Electron pairs involved in bond formation exhibit structurally
stable configurations. Electrons within the human body form compounds and pairs due
to the bonds they establish. Upon bond disruption, electrons either disperse to other
atoms or remain on a single atom.
When electrons remain localized on a single atom, ions are formed, whereas their
dispersion to different atoms results in the formation of free radicals. Unpaired
electrons possess unexpectedly high energy levels, which can lead to the disruption of
paired electrons in other molecules.
Antioxidants are substances that inhibit or decelerate undesirable oxidation reactions.
They neutralize free radicals, thereby preventing cellular damage induced by oxidative
stress. Consequently, due to their role in naturally ameliorating various medical
conditions caused by free radicals within the human body, there has been an observed
increase in the consumption of antioxidant-rich foods. However, excessive intake of
artificially synthesized antioxidants may lead to adverse health conditions, such as
cancer. Therefore, antioxidants derived from natural sources are favored.
Phenolic compounds are one of the secondary metabolites synthesized during the
normal development of plants. These phenolic substances typically possess one or
more hydroxyl groups attached to an aromatic ring and can be categorized as metabolic
products with varied structures and functions.
Phenolic compounds present in foodstuffs are predominantly found in fruits; however,
their presence can vary depending on the type of fruit. Even within the same fruit
species, the concentration of phenolic compounds can be influenced by multiple
factors, including species, growth season, environmental conditions, climatic factors,
soil type, geographical location, and ripeness.
The term 'oxidative stability' in foods can be modified by phenolics, which can alter
attributes such as bitterness, color, taste, pungency, flavor, and aroma. The presence
of these compounds significantly impacts the sensory characteristics and shelf life of
foods, thereby enhancing consumer acceptance and nutritional value.
The hydroxylation pattern around the aromatic ring dictates antioxidant activity. The
capacity to act as a hydrogen donor and inhibit oxidation is directly proportional to the
number of hydroxyl groups present on the phenolic rings. The fundamental attribute
of a molecule or antioxidant system is its ability to maintain equilibrium and,
consequently, contribute to the reduction of oxidative stress-induced damage in human
metabolism by disrupting or recognizing oxidative propagation chains.
Phenolic compounds found in foods, particularly in fruits, have notable health benefits
despite lacking direct nutritional functions. Flavonoids and other plant polyphenols are
significant antioxidants due to their high redox potentials. Grape seed extract,
recognized for its potent antioxidant properties, neutralizes free radicals while
exhibiting cytotoxic effects against cancerous cells without harming healthy cells. It
has been reported to prevent prostate cancer, and gallic acid (GA) has demonstrated
significant effects against leukemia.
Additionally, GA possesses protective properties against chronic cardiovascular
diseases and cancer. Phenolic compounds in foods, despite lacking nutritional
functions, have positive health benefits. Due to their high redox potentials, flavonoids
and other plant polyphenols serve as very important antioxidants. The antioxidant
effects of phenolic compounds are explained by their ability to bind with metals, form
chelates, and inactivate spesifik enzymes.
The most common group of naturally occurring phenolic substances is flavonoids.
Other phenolic compounds include simple phenols, phenolic acids (such as benzoic
and cinnamic acids), coumarins, stilbenes, hydrolyzable and condensed tannins,
lignans, and lignins. These diverse phenolic compounds are found across a wide range
of plants and thus plant-based foods, providing numerous beneficial effects on human
health.
Gallic acid (GA), a notable antioxidant present in natural samples, is a member of the
polyphenol group and is sometimes referred to as 3,4,5-trihydroxybenzoic acid.
Comprising C6H2(OH)3CO2H molecules, gallic acid is a yellow-white crystalline
substance. It is highly soluble in water and dissolves in ether, alcohol, and glycerol,
but is insoluble in benzene and chloroform.
Gallic acid can be found in various fruits such as pomegranate, banana, blackberry,
blueberry, avocado, currant, mango, mulberry, and sumac, as well as in plant products
like green tea, sumac, walnut, oak, chestnut trees, and certain barks.
In recent years, various chromatographic techniques and methods such as
chemiluminescence spectrophotometry have been employed for the detection of
phenolic compounds. The electrochemical detection of gallic acid occurs due to the
release of electrons during its oxidation. Gallic acid is utilized as a standard compound
for determining the phenolic content in diverse biological food, juice, and plant
samples.
In this study, electrochemical sensors for the detection of gallic acid were developed
using glassy carbon electrodes (GCE) in conjunction with semiconductors such as zinc
oxide (ZnO), zinc sulfide (ZnS), and zinc selenide (ZnSe). Gallic acid detection was
performed using these electrochemical sensors with GCE/ZnS, GCE/ZnSe, and
GCE/ZnO working electrodes in solutions containing various concentrations of GA in
0.1 M phosphate buffer (PBS) (pH 7.4).
The developed GA sensors exhibited a broad linear response range from 10 μM to
1000 μM. The electrochemical sensors developed with GCE/ZnS, GCE/ZnSe, and
GCE/ZnO electrodes demonstrated high sensitivity, excellent reproducibility, high
selectivity, and long-term stability for GA detection.